groups: do not exit early

groups: do not exit early

Most programs take care to operate on all command-line-specified
operands before exiting.  That is an important feature that allows
to identify all problems with the first run.  However, groups would
exit upon the first problematic user name.
Bug introduced via commit v6.10-56-g167b8025ac.
* src/groups.c (main): Do not exit immediately upon error.
* tests/misc/groups-process-all.sh: New file. Test for this.
* tests/local.mk (all_tests): Add it.
* NEWS (Bug fixes): Mention this.