Supports .mpdignore on NFS/SMB and others (closes #290).

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

Closes #169

Signed-off-by: FlashSystems <developer@flashsystems.de>

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

Wildcard matches are directly applied to all filenames in
subdirectories without any attempt at matching relative paths.

This change is based on the following feature request:

  http://bugs.musicpd.org/view.php?id=3729

std::auto_ptr is deprecated, and std::unique_ptr is much better
anyway.

Check for 0 in "device" and "inode" instead.

Remove the C list_head library and use type-safe C++ instead.

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.

Replaces map_to_relative_path() from Mapper.cxx.