sysutils/screen*: Rename screen and introduce a meta port

This is in preparation for the import of screen 5.0.0. I've tried
screen-devel (5.0.0 prelrelease). There were some bugs and
inconsistencies with screen 4.9.1. So we move screen to screen49
to maintain a version of screen that is known to work.

This implements the concept of a master screen port by renaming it to
screen49 and creating a slave port called screen. The idea of this is
similar in concept used with the cfengine or krb5 family of ports.
Users should never need to change port origins when a new stable
version of screen is committed.

People can install the screen port or package as before. However if they
choose to use the other screen port/package they may remove screen and
install the versioned "master" port/package directly.

This is in preparation for import of screen 5.0.0 into the ports
collection. Unfortunately my initial testing of screen 5.0.0 using the
screen-devel port uncovered an annoying (to me) bug, which I have opened
an issue with our upstream. I have not found the time to test their
patches enough to feel comfortalbe replacing the existing screen 4.9.1
with 5.0.0. When 5.0.0 will be imported as sysutils/screen50, it will
not become the default master port until we believe it won't become
a source of many bugzilla bugs.