We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

An ino_t is usually a 64 bit integer, and some file systems (such as
Linux's kernel NFS client) really uses the upper 32 bit.  This can
lead to false positives in the directory loop detection in
FindAncestorLoop().  Increasing these two attributes (in
StorageFileInfo and Directory) to 64 bit adds little overhead, but
makes the check a lot safer.

Eliminate dependency on io_thread_get().

If an error has already been set by f.directory->storage->GetInfo(),
don't set it again.

It was used in a wrong way, which did not deal with errors
consistently.  And if that's wrong, there is no need for FindStorage()
at all - let's remove it and the confusion around it.

default_value defaults to nullptr.

The Connect method can be called between Schedule and lock. In that case, when
locked, the state is already set to CONNECTING of READY and the condition won't
be signaled anymore.

Replaces StatFile(), with a portable data object.

Return false on error, not nullptr.

Fixes bogus assertion failure.