pkg: use specific CONFSNAME_${file} for FreeBSD.conf

Setting CONFSNAME directly is a little more complicated for downstream
consumers, as any additional CONFS that are added here will inherit the
group name by default. This is perhaps arguably a design flaw in CONFS
because inheriting NAME will never give a good result when additional
files are added, but this is a low-effort change.

While we're here, pull FreeBSD.conf.${branch} out into a PKGCONF
variable so one can just drop a new repo config in entirely with a new
naming scheme. CONFSNAME gets set based on chopping anything off after
".conf", so that, e.g.:

• FooBSD.conf => FooBSD.conf
• FooBSD.conf.internal => FooBSD.conf

Reviewed by: bapt, manu
Differential Revision: https://reviews.freebsd.org/D28767