(vc-do-command): Change RCS handling so rcsdiff won't strip

(vc-do-command): Change RCS handling so rcsdiff won't strip
away relative-pathname information.  This function no longer sets the
default directory.  Also, mark the *vc* output buffer unmodified.
(vc-revert-buffer1): Handle font-lock mode correctly.
(vc-diff, vc-print-log): vc-do-command no longer sets the default
directory, but doing so	is advantageous for these cases.
(file-executable-p-18):	Better portability to Emacs 18.
(vc-directory-exclusion-list, vc-file-tree-walk-internal):
Implement the new variable vc-directory-exclusion-list to prune
tree walks.  Initial value tells it to ignore SCCS and RCS subdirectories.