Clones the git repo into $(SBBSIDR)/git building everything there and then
copying the run-time dirs (e.g. ctrl, exec, text, xtrn, web) to the
$(SBBSDIR). This means a sysop cannot simply "git-update" (there is no
such thing) the exec or xtrn dirs as someone might have done previously with
CVS. Update instructions on the wiki will be forthcoming.

Removed the NOCVS option (if the workdir already exists, no clone occurs).
Changed CVSTAG to just TAG. It does not create a local branch for the
checked-out tag (perhasp it should).
Removed the INSTALL type (only CLASSIC was installed anyway), that should
be easy to restore if/when UNIX-style install is ever implemented.