Don't declare things in more than one place. Move some declarations

between headers.  Code is now fully prototyped and compiles cleanly
with gcc -Wstrict-prototypes -Wmissing-prototypes -Wnested-externs
-Wredundant-decls.  Closes #723788.

src/lib/common/mailbox.c

@@ -33,6 +33,7 @@
 
 #include "misc.h"
 #include "tel.h"
+#include "optlist.h"
 #include <stdio.h>
 
 s_char *