s4-selftest: added --screen option for test

s4-selftest: added --screen option for test

you can now do:

  make test TESTS="some test" SCREEN=1

while in GNU screen, and all the samba servers will launch in their
own new screen, named after the server name.

You can also do:

  make test TESTS="some test" SCREEN=1 VALGRIND_SERVER=1

to run valgrind on each samba server, or

  make test TESTS="some test" SCREEN=1 GDBTEST=1

to run gdb on each server