sysutils/screen: Disable multiuser mode by default

Multiuser mode is a handy way to share a screen among people who do
not reside in the same location. Unforutnately it requires that screen
be setuid root. GNU screen has had a number of CVEs over the years.
See https://www.cvedetails.com/vulnerability-list/vendor_id-72/\
product_id-1860/GNU-Screen.html. Removing the setuid bit mitigates this
at the expense of breaking the multuser feature.

Red Hat removed GNU screen's setuid bit over a dozen years ago. Their
rationale is documented in their bugzilla bug 580339, where they stated
that most users don't use the multiuser feature. (Personally, I'm the only
person I know of who uses that feature.)

Users who use the multuser feature should enable the MUILTUSER option
prior to building screen or using poudriere-options. Alternatively, users
can chmod the setuid bit on when needed.

PR: 262903
Submitted by: david@isnic.is (mostly)
Reported by: david@isnic.is