pkg runs install scripts with PKG_ROOTDIR=/, so prepending it to
absolute paths printed "//usr/local/etc/mail/mailer.conf" in the
activation message. MC_BASE relied on the trailing slash instead and
would break with "pkg -r /some/root".
Strip the trailing slash once and always join with an explicit "/".
Also included as a separate commit: reorder ALL_TARGET and PORTSCOUT
to make portclippy happy (no functional change).