sysutils/screen50: Introduce the new screen 5.0.0

Screen 5.0.0 is the next major release of GNU Screen. Unfortunatly
while testing sysutils/screen-devel I discovered bugs with its
hardstatus implementation. Existing hardstatus failed to work
correctly. I had opened a bug with the GNU Screen upstream. They
had fixed one of the bugs however the last time I tested screen-devel,
some status strings were still unavailable, not to mention that the
hardstatus string syntax has changed.

Therefore I don't believe screen50 should replace screen49 as our
default GNU screen until a) some of the issues are resolved and
b) we the FreeBSD community have had a little more experience with
it.

People who wish to use screen50 will need to uninstall screen
(currently a meta port pointing to screen49) and install screen50.

Any bugzilla bugs will likely spawn an upstream bug report by
myself.