This replaces the attributes bits, channels, sampleRate.

Fix some gcc warnings by using unsigned where appropriate.  Declare
numfrequencies as "const", and replaced the magic number 12 with a
sizeof.

Again, a plugin which was disabled in my test environment and suffered
from compilation errors.

This replaces the separate properties channels, sampleRate, bits.

A frame contains one sample per channel, thus it is sample_size *
channels.  This patch includes some cleanup for various locations
where the sample size for 24 bit audio was still 3 bytes (instead of
4).

The last bit of CamelCase in audio_format.h.  Additionally, rename a
bunch of local variables.

"bits" and "channels" cannot be negative.

There is only once update thread at a time.  Make the "modified" flag
global and remove the return values of most functions.  Propagating an
error is only useful for updateDirectory(), since updateInDirectory()
will delete failed subdirectories.

When a song file was not modified, MPD printed the debug message "not
a directory or music", because the first "if" branch did not return.

When the update queue is full, directory_update_init() did not free
the path argument.

Since the return value cannot be -1 anymore, we can make it unsigned.

The documentation for directory_update_init() was incorrect: a job ID
must be positive, not non-negative.  If the update queue is full and
no job was created, it makes more sense to return 0 instead of -1,
because it is more consistent with the return value of isUpdatingDB().

When the update task is idle, there is no need to check for deleted
songs.  Return early from reap_update_task().

pthread_join() expects a "pointer to a pointer" parameter, but it got
a "pointer to an enum".  On AMD64, an enum is smaller than a pointer,
leading to a buffer overflow.

Removed the local variable "was_empty": don't remember if the
directory is new.  Always call removeDeletedFromDirectory().

In updateInDirectory(), add new directories immediately and
delete them when they turn out to be empty.  This simplifies the code
and allows us to eliminate addSubDirectoryToDirectory().

The recursive checks were performed in several functions, and
sometimes a directory was checked twice.

When reading a new directory, copy the stat data (which we have
anyway) to the directory struct.  This may save a stat() in the
future.

Pass a pointer to the stat struct to more functions.

updatePath() duplicated a lot of code from the more generic
updateInDirectory().  Eliminate most of updatePath() and call
updateInDirectory().

If the user requests database update during startup, call
directory_update_init().  This should be changed to fully asynchronous
update later.

For this to work, main_notify has to be initialized before db_init().

The stat struct isn't going to be modified, make it const.

update_task() already checks if it has got a root path.  Extend this
check and in turn remove a check in the inner function updatePath().

Search'n'replace typo..

directory_update_init() has to be called with a path that is already
sanitized.  Don't call sanitizePathDup() again in updatePath().

The algorithm in addDirectoryPathToDB() can be simplified further if
it is combined with the function addParentPathToDB().  Since there is
no other caller of addDirectoryPathToDB(), we can do that.  This saves
another large stack buffer.

This recursive function is very dangerous because it allocates a large
buffer on the stack in every iteration.  That may be misused to
generate a stack overflow.

When a directory cannot be updated, there must be something wrong with
it, and the database contains stale data.  Remove it.

The branching looks a bit complicated in addDirectoryPathToDB() -
improve its readability by moving code to a simplified separate
function.

When the root directory fails to update, its contents are invalid.
Clear it then.

When a directory failed to update, it was removed from the database,
without freeing all children and songs (memory leak), and without
locking (race condition).  Introduce the functions clear_directory()
and delete_directory(), which do both.

Use updateInDirectory() instead of addToDirectory().  Eliminate a
duplicate stat() in updateInDirectory() by calling song_file_update()
directly.

Don't use db_get_directory() and traverse the full path with every
directory being loaded.  Just see if the current parent contains the
entry.  Everything else would be invalid anyway..

A manipulated database could trigger an assertion failure, because the
parent didn't match.  Do a proper check if the new directory is within
the parent's.  This uses FATAL() to bail out, so MPD still dies, but
it doesn't crash.

Some tiny utilities... wrappers like these may become helpful when we
introduce locking.

dirvec_find() does not modify the object, thus it should get a const
pointer.