vc-print-log should be pickier about using log-view-mode

[Jonathan Kamens]

In GNU Emacs 21.1.1 (i386-redhat-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2001-12-06 on stripples.devel.redhat.com

vc-print-log, at least as of revision 1.311 of vc.el, always uses
log-view-mode if it's bound.  The problem with this is that
log-view-mode only supports RCS, CVS and SCCS logs, so vc-print-log
will behave incorrectly if another SCM system is integrated into the
new vc.el.

Either vc-print-log should only use log-view-mode if the backend is
RCS, CVS or SCCS, or a new entry point needs to be added to vc-*.el,
something like vc-BACKEND-use-log-view-p, so that newly added SCM
systems can tell vc.el whether log-view-mode will work with them.