added function to check if one option is a grouped short option
added function for parsing grouped short option (without body)
added logic to parse grouped short opts, too
added short option group parsing algo, untested
added clean task
fixed check for short option grouping
added -O2 only for "all" task, not for testing task
is_grouped_short_option is inlined, and outsourced length check
Fixed check if configured option is grouped short

This check was fixed by this commit, as a configured option which can be part of
a grouped short option is always "-<character>", so its length is 2.

Comments for this were added.
The new offset must be zero

as the for() loop adds one, too. So we have to return 0 here.
Merge branch 'feature-short_option_grouping'