Use shells(8) from sysutils/etcutils to update /etc/shells instead of
hardcoding the logic into the pkginstall scripts.  As discussed in
tech-pkg@.

Note: The current pkginstall/shell code is overly complicated.  It looks
like it can be simplified but, at the moment, given that I do not understand
the need for such complexity, I'm just doing this tiny change.

Note 2: The ability to update /etc/services, which was also discussed, will
come later once this change proves to be stable.

(jmmv)