tftpd: explicitly set egid after dropping supplemental groups

[kevans]

Summary

tftpd seems to be the last program in base that implicitly relies on
setgroups() to set the egid. This is a security landmine in portable
software as most operating systems don't behave this way, so do an
explicit setgid() in case the kernel doesn't set it already.

[olce]

libexec/tftpd/tftpd.c

354

Let's be portable right now and leverage the already-existing special case we have to remove all supplementary groups.

[kevans]

Clear the supplemental groups entirely

The following has been added to the commit message:

While we're here, FreeBSD's setgroups() has supported nominally clearing
all supplemental groups since 1997.  It still leaves the egid in our
cr_groups[0] because we don't have an out-of-band way to store the egid,
and on other systems it'll clear the supplemental group entirely as one
would want.