Try map_fs_to_utf8() first, and fall back to Path::ToUTF8() for
absolute paths.

Consistently use std::string in LoadPlaylistFile().

Avoid the conversion to an allocated char*, let the caller decide.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

.. instead of rolling own classes.

Use the C++ API.

g_file_error_quark() is meant to be used with the GFileError enum
which does not correspond with errno, but must be converted with
g_file_error_from_errno().

At the same time, this removes g_strerror() use for
g_file_error_quark().

Allow the plugin to allocate the GetSong() return value.

Improve error reporting and handling.  command.c gets the new function
print_error(), which sends a GError to the client.

Only used there.

Don't look up songs in the database, no caller needs this.

MPD doesn't have child processes anymore, and thus we're not expecting
to receive SIGCHLD very often.  Since hard disk access isn't
interrupted by signals anyway, we don't need those excessive checks.