player/misc.c: allow both --length and --end to control play endpoint

Most options that change the playback endpoint coexist and playback
stops when it reaches any of them. (e.g. --ab-loop-b, --end, or
--chapter). This patch extends that behavior to --length so it isn't
automatically trumped by --end if both are present. These two will
interact now as the other options do.

This change is also documented in DOCS/man/options.rst.