Don't call setgrent() in an unportable way.

For FreeBSD 12, I'm considering updating setgrent() to have a function
prototype that conforms to POSIX. FreeBSD seems to be the only operating
system that lets setgrent() return an integer. It's also inconsistent
with setpwent().

It looks like our libcasper depends on setgrent() returning an integer.
Get rid of that.

Reviewed by: oshogbo
Differential Revision: https://reviews.freebsd.org/D6659