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 */

include/unixconf.h

@@ -193,7 +193,9 @@
 
 /* If SIMPLE_MAIL is defined, the mail spool file format is
    "sender:message", one mail per line, and mails are
-   read within game, from demon-delivered mail scrolls */
+   read within game, from demon-delivered mail scrolls.
+   The mail spool file will be deleted once the player
+   has read the message. */
 /* #define SIMPLE_MAIL */
 
 #ifndef MAILCKFREQ
@@ -203,6 +205,16 @@
 
 #endif /* MAIL */
 
+/* If SERVER_ADMIN_MSG is defined and the file exists, players get
+   a message from the user defined in the file.  The file format
+   is "sender:message" all in one line. */
+/* #define SERVER_ADMIN_MSG "adminmsg" */
+#ifndef SERVER_ADMIN_MSG_CKFREQ
+/* How often admin message file is checked for new messages, in turns */
+#define SERVER_ADMIN_MSG_CKFREQ 25
+#endif
+
+
 /*
  * Some terminals or terminal emulators send two character sequence "ESC c"
  * when Alt+c is pressed.  The altmeta run-time option allows the user to