Add missing /usr/lib/debug directories

Directories for /usr/lib{,32}/{i18n,private} were missing from the mtree
file, which caused installworld to install the files that should be in
the directory as the name of the directory.
Do not delete the group wheel when bad argument is passed to pw groupdel -g

Check that the -g argument is actually a number, if not report an error.
This argument is converted without checking with atoi(3) later so without this
check it converts any alpha entries into 0 meaning it deletes the group wheel

Add a regression test about it

PR:		90114
Reported by:	bkoenig@cs.tu-berlin.de
MFC after:	1 week