Type-safe wrapper for uri_get_suffix().

Don't pass Path::c_str() to tag_stream_scan() which requires a UTF-8
string.

Reduce some unnecessary overhead.

Eliminate duplicate code.

Replaces StatFile(), with a portable data object.

Previously, MPD printed the time stamp without printing the file
name.  That made no sense.

PathExists() should better do what CheckAccess() does, and
CheckAccess() doesn't do what its name implies.

Handles abstract sockets on Linux.