decoder/gme: fix song duration

The unit of gme_info_t::length is milliseconds, not centiseconds.
doc/protocol: fix description of "stats" response

Fix incorrect description of the "songs" field and add missing
"albums" field.

Signed-off-by: Joachim Fasting <joachifm@fastmail.fm>
output/{alsa,oss}: add assertions
output/alsa: fix endless loop at end of file in dsd_usb mode
PlaylistControl: move code to new method SeekSongOrder()
PlaylistControl: use SeekSongOrder(current) to keep current song

The "current" attribute is a "song order", not a "song position".
This is usually the same - except in random mode.  Fixes Mantis ticket
0004073.
release v0.18.13
Merge tag 'v0.18.13'