mixer: Allow arbitrary relative time for sleep subcommand.

mixer: Allow arbitrary relative time for sleep subcommand.

If --wake-hour is not given, the wake time defaults to now + nine
hours.  This is arbitrary, and the constant 9 was hardcoded with no
way to specify it at the command line or in the config file.

This commit replaces --wake-hour and --wake-min by --wake-time whose
argument must be given in HH:MM syntax. An optional leading '+'
character allows to specify a relative offset.