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.

include/extern.h

@@ -1062,6 +1062,7 @@ E void FDECL(nocmov, (int x, int y));
 #ifdef UNIX
 E void NDECL(getmailstatus);
 #endif
+E void NDECL(ck_server_admin_msg);
 E void NDECL(ckmailstatus);
 E void FDECL(readmail, (struct obj *));
 #endif /* MAIL */