New API fm_uri_subpath_to_native_subpath().

It unescapes URI subpath and then converts is into local filename encoding.
This API should be used on file copying from remote to native file.
URI file://somehost/ should be not treated as local file I believe.
Fix FmPath references in _fm_path_init().

Also update some comments in code, documantation related.
Correctly convert filename encoding on copy between native and virtual FS.
Don't go beyond question mark on search:// URI in fm_path_new_for_display_name()

The path string still may contain slashes but trying to resolve them
may corrupt it (e.g. if it contains /../ as search pattern) therefore
nothing beyond the '?' should be checked.