Convert manpage to mandoc and fix usage

In all honor, the previous usage was formally more correct, but for the
sake of consistency across all the tools having the v-flag, I separated
it from the command-string.

Also, make use of the mandoc macros for the manpage. This makes it
easier to maintain, extend and change in the future.

slock.c

@@ -282,7 +282,7 @@ lockscreen(Display *dpy, int screen)
 static void
 usage(void)
 {
-	die("usage: slock [-v | cmd [arg ...]]\n");
+	die("usage: slock [-v] [cmd [arg ...]]\n");
 }
 
 int