tftpd: explicitly set egid after dropping supplemental groups

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.

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.

Reviewed by: allanjude (previous version), des, olce
Differential Revision: https://reviews.freebsd.org/D51149