Generate an RFC822 (generic) Message-ID header field when importing msgs

(using -i). These aren't quite as good as the IDs we can generate when we
have access to the BBS configuration (scfg_t), but it's decently unique and
identifiable.

src/sbbs3/smbutil.c

@@ -165,6 +165,32 @@ ulong lf_expand(uchar* inbuf, uchar* outbuf)
 	return(j);
 }
 
+char* gen_msgid(smb_t* smb, smbmsg_t* msg, char* msgid, size_t maxlen)
+{
+	char* host = getenv(
+#if defined(_WIN32)
+		"COMPUTERNAME"
+#else
+		"HOSTNAME"
+#endif
+	);
+	if(host == NULL)
+		host = getenv(
+#if defined(_WIN32)
+		"USERNAME"
+#else
+		"USER"
+#endif
+	);
+	safe_snprintf(msgid, maxlen
+		,"<%08lX.%lu.%s@%s>"
+		,msg->hdr.when_imported.time
+		,smb->status.last_msg + 1
+		,getfname(smb->file)
+		,host);
+	return msgid;
+}
+
 /****************************************************************************/
 /* Adds a new message to the message base									*/
 /****************************************************************************/
@@ -325,6 +351,8 @@ void postmsg(char type, char* to, char* to_number, char* to_address,
 		bail(1); 
 	}
 
+	smb_hfield_str(&msg, RFC822MSGID, gen_msgid(&smb, &msg, str, sizeof(str)-1));
+
 	if(mode&NOCRC || smb.status.max_crcs==0)	/* no CRC checking means no body text dupe checking */
 		dupechk_hashes&=~(1<<SMB_HASH_SOURCE_BODY);