read_simplemail() when SERVER_ADMIN_MSG is defined

Some warnings were mentioned

Add a prototype ahead of the function
Use a non-const copy of SERVER_ADMIN_MSG

quick-tested by:
- uncommenting the following in include/unixconf.h
  /* #define SERVER_ADMIN_MSG "adminmsg" */
- building NetHack
- creating a test message:
  echo "server_admin: system is going down at 2 pm" >~/nh/install/games/lib/nethackdir/adminmsg
- playtested and received the desired message

src/hacklib.c

@@ -1221,12 +1221,12 @@ strbuf_nl_to_crlf(strbuf_t *strbuf)
 }
 
 char *
-nonconst(const char *str, char *buf)
+nonconst(const char *str, char *buf, size_t bufsz)
 {
     char *retval = emptystr;
 
     if (str && buf)
-        if ((int) strlen(str) < BUFSZ - 1) {
+        if (strlen(str) < (bufsz - 1)) {
 	    Strcpy(buf, str);
             retval = buf;
         }