libsubcmd: Fix OPTION_GROUP sorting

The OPTION_GROUP option type is a way of grouping certain options
together in the printed usage text.  It happens to be completely broken,
thanks to the fact that the subcmd option sorting just sorts everything,
without regard for grouping.  Luckily, nobody uses this option anyway,
though that will change shortly.

Fix it by sorting each group individually.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Link: https://lkml.kernel.org/r/e167ea3a11e2a9800eb062c1fd0f13e9cd05140c.1650300597.git.jpoimboe@redhat.com