Add server admin messaging functionality

It's occasionally important for public servers to notify
all the players. Sending a mail is not reliable, as not everyone
wants to break conduct, or have mail on.

This adds a compile-time defined filename, which NetHack
will monitor. The contents of the file are in the same
format as SIMPLE_MAIL: "sender:message" on one line.

src/allmain.c

@@ -455,6 +455,7 @@ boolean resuming;
                 rhack(save_cm);
             }
         } else if (multi == 0) {
+            ck_server_admin_msg();
 #ifdef MAIL
             ckmailstatus();
 #endif