groupmember.9: remove the man page

[mhorne]

Summary

It really doesn't offer anything over the function definition itself,
and it has no external references. Add a very minor clarification to the
top-level comment there and drop this page.

[mhorne]

Seems reasonable?

[rpokala]

Maybe add an entry for groupmember() to ucred.9, along with other (struct ucred)-related functions? crsetgroups() is there, for example.

(If you do that, you should also add it and crsetgroups to the .Sh NAME section.)

[kib]

sys/kern/kern_prot.c

1281

Fix style while there?

1297

[mhorne]

Someone wants to make this page better: D40634.