Renove duplicate code.
API simplification: remove vc-workfile-unchanged-p from pubic methods.

* vc/vc.el, vc-hooks.el, and all backends: API simplification;
vc-workfile-unchanged-p is no longer a public method (but the RCS and
SCCS back ends retain it as a private method used in state
computation). This method was redundant with vc-state and usually
implemented as a trivial call to same. Fixes the failure mode
described in bug#694.
The vc-mistrust-permissions configuration variable is gone.

* vc/vc-cvs.el, vc/vc-hooks.el, vc/vc-rcs.el, vc/vc-sccs.el: Eliminate
vc-mistrust-permissions.  It was only relevant to the RCS and SCCS
back ends and defaulted to t. Code now always mistrusts permissions -
by actual measurement the effect on performance is negligible. As a
side effect bug#11490 is now irrelevant.