Fix up setgrent(3) to have a POSIX-compliant prototype.

Fix up setgrent(3) to have a POSIX-compliant prototype.

Just like with freelocale(3), I haven't been able to find any piece of
code that actually makes use of this function's return value, both in
base and in ports. The reason for this is that FreeBSD seems to be the
only operating system to have such a prototype. This is why I'm deciding
to not use symbol versioning for this.

It does seem that the pw(8) utility depends on the function's typing and
already had a switch in place to toggle between the FreeBSD and POSIX
variant of this function. Clean this up by always expecting the POSIX
variant.

There is also a single port that has a couple of local declarations of
setgrent(3) that need to be patched up. This is in the process of being
fixed.

PR:		211394 (exp-run)