Changed the GECOS-field '&' translation behavior to be controlled by the BSD42

#define rather than GCOS_HACK, since it's apparently always appropriate on OSes
where BSD42 is #defined, and never appropriate on any other OSes.  Thanks to
Kimmo Suominen for responding to my "What is this code here for?" comment in
mts.c and explaining the feature.

Also added ULTRIX 4.2A to the list of OSes that have an initgroups() function
but no prototype in the system headers.