Move code from Path::RelativeFS() and make it generic.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Move code from map_fs_to_utf8().

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

The function path_set_fs_charset() is only used in path.c.

Don't use fixed static buffers.  GLib allocates a new string for us
anyway, let's just return this one instead of copying it.

Use GLib's g_build_filename() instead of pfx_dir().

Only include headers which are really needed.  os_compat.h aimed to
make MPD easily portable, but was never actually made portable.