Closes https://github.com/MusicPlayerDaemon/MPD/issues/841

Mounting one storage URI twice on different mount points can lead to
conflicts with the database cache file, and it doesn't make a lot of
sense.

But most importantly, our udisks storage plugin will unmount the disk
from the kernel VFS, and if two exist, they will compete with each
others.  We could (and should) fix this in the udisks storage plugin,
but for now, this workaround is good enough (and useful).

When mounting something over a directory that is already a mount
point, CompositeStorage::Mount() silently overwrites the previously
mounted storage, disposing it.  After that, SimpleDatabase::Mount()
will fail and handle_mount() will roll back the
CompositeStorage::Mount() command, effectively unmounting what was
there before (and also leaking memory).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/918

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

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

These events are relevant for all partitions.

Eliminate global variables.

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().

Lists files and directories.  Supports storage plugins.

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

Add the new idle event to Idle.hxx/Idle.cxx.