Add new I-restriction, to disallow receipt of Internet mail to specific users

(mail received from authenticated SMTP clients is still allowed)

Previously, the M-restriction also prevent receipt of Internet mail from
authenticated SMTP clients, so separate that restriction into the new 'I'
restriction.

This fixes issue #865

src/sbbs3/mailsrvr.c

@@ -4899,8 +4899,8 @@ static bool smtp_client_thread(smtp_t* smtp)
 				continue;
 			}
 			if (cmd == SMTP_CMD_MAIL) {
-				if ((user.rest & FLAG('M')) && relay_user.number == 0) {
-					lprintf(LOG_NOTICE, "%04d %s %s !M-restricted user-recipient #%u (%s) cannot receive unauthenticated SMTP mail"
+				if ((user.rest & FLAG('I')) && relay_user.number == 0) {
+					lprintf(LOG_NOTICE, "%04d %s %s !I-restricted user-recipient #%u (%s) cannot receive unauthenticated SMTP mail"
 					        , socket, client.protocol, client_id, user.number, user.alias);
 					sockprintf(socket, client.protocol, session, "550 Closed mailbox: %s", rcpt_to);
 					stats.msgs_refused++;

text/menu/restrict.asc

 A=ANSI              H=                  O=Post w/RealName   V=Voting
-B=Beep              I=                  P=Post on any sub   W=Auto-message
+B=Beep              I=RX Internet Mail  P=Post on any sub   W=Auto-message
 C=Chat              J=Quoting           Q=QWK Network Node  X=External Programs
 D=Download          K=Read Mail Sent    R=Remove Files      Y=
 E=E-mail            L=Logon one/day     S=Email Sysop       Z=
-F=Forward mail      M=Networked Mail    T=Transfers
+F=Forward mail      M=Send NetMail      T=Transfers
 G=Edit Defaults     N=Networked Subs    U=Upload