On BSD/OS, at least, inc.c needs all "setgid" changed to "setegid" to work
with MAILGROUP defined, because once you setgid to the user's gid, you aren't
allowed to change back, because setgid changes real, effective, and "saved"
gid. You can only setgid to a value which is still stored in one of those.
With it changed to "setegid", inc creates files with my group id in my Mail
directory, but has group mail privs to /var/mail. I am pretty sure this is
correct almost everywhere, although it wouldn't surprise me if just using
setgid worked on some systems.
I can submit a detailed patch, but it's pretty much
:.,$s/setgid/setegid/g
in inc.c from the line after the comments describing the "setgid" logic.
-s
