xinstall: Fix broken memmove with -t

memmove moves a number of bytes, not pointers, so if you passed a number
of arguments that is larger than the pointer byte size, you could
end up crashing or skipping the install of a file and installing another
twice.

Also, argv was never decreased to match the moved arguments, so the -t
parameter was added in the NULL argv slot.

xinstall.c

@@ -219,7 +219,7 @@ main(int argc, char *argv[])
 		mode = parsemode(mflag, mode, 0);
 
 	if (tflag) {
-		memmove(argv - 1, argv, argc);
+		argv = memmove(argv - 1, argv, argc * sizeof(*argv));
 		argv[argc++] = tflag;
 	}
 	if (tflag || argc > 2) {