Switch xcscope.el to a separately-maintained version.

The version of xcscope.el bundled with cscope does not work with emacs
27 as it uses a deprecated function. Switch to using a newer fork of
xcscope.el which works with emacs 27 and includes several other fixes
such as working over tramp-mode. Since the newer fork is hosted
independently, add it as a new port which depends on cscope.

PR: 251029
Reviewed by: maintainer (devel/cscope)
Approved by: swills
Differential Revision: https://reviews.freebsd.org/D27172