This optimization is useless because sane pthread_cond_signal()
implementations check the number of waiters and do not invoke a system
call if there are none.

And allow "Z" suffix after date.

Prepare for adding more flexible parsing.

Move code from Parser.cxx.

This is recommended by the strptime() manpage, because strptime() does
not initialize/set attributes which were not specified in the format
string.

Simplify the plugin by using a trivial relative URI; later,
playlist_check_translate_song() will set the correct full URI.

Fixes -Wreturn-std-move (clang 8).