(vc-backend-checkout): Add if-statements to the shell cmds

(vc-backend-checkout): Add if-statements to the shell cmds
to discard the extra arg some shells make.

(vc-rename-file): Add autoload cookie.

(vc-steal-lock): (1) Function should be non-interactive, because it
doesn't work when called directly by the user. (2) Don't display
revision "nil" when stealing lock if revision is unknown.  (3) Put
"Stolen lock on <file>" instead of just the file name in the Subject
of the notification message.