Now, we can run the PPP interface in an rdomain and get a default route
installed.

Here is an improved version compared to yesterday's diff, fixing the warning
message about mismatch on rdomain.

```
# ifconfig ppp0 rdomain 5 up
# ifconfig em0 192.168.8.2/30 up
# route add 10.42.0.240 192.168.8.1
# rcctl start xl2tpd
# echo c l2tp > /var/run/xl2tpd/l2tp-control

$ ping -V5 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
64 bytes from 8.8.8.8: icmp_seq=0 ttl=119 time=71.606 ms
64 bytes from 8.8.8.8: icmp_seq=1 ttl=119 time=64.730 ms
```

OK ?

Index: sys/net/ppp_tty.c
===================================================================
RCS file: /cvs/src/sys/net/ppp_tty.c,v
diff -u -p -r1.54 ppp_tty.c
--- sys/net/ppp_tty.c	2 Jan 2022 22:36:04 -0000	1.54
+++ sys/net/ppp_tty.c	30 Jun 2024 15:43:19 -0000
@@ -371,6 +371,7 @@ pppwrite(struct tty *tp, struct uio *uio
     bcopy(mtod(m0, u_char *), dst.sa_data, PPP_HDRLEN);
     m0->m_data += PPP_HDRLEN;
     m0->m_len -= PPP_HDRLEN;
+    m0->m_pkthdr.ph_rtableid = sc->sc_if.if_rdomain;
     return sc->sc_if.if_output(&sc->sc_if, m0, &dst, NULL);
 }

Index: usr.sbin/pppd/sys-bsd.c
===================================================================
RCS file: /cvs/src/usr.sbin/pppd/sys-bsd.c,v
diff -u -p -r1.33 sys-bsd.c
--- usr.sbin/pppd/sys-bsd.c	19 Feb 2024 16:39:03 -0000	1.33
+++ usr.sbin/pppd/sys-bsd.c	30 Jun 2024 15:43:21 -0000
@@ -118,6 +118,7 @@ static int initdisc = -1;	/* Initial TTY
 static int initfdflags = -1;	/* Initial file descriptor flags for ppp_fd */
 static int ppp_fd = -1;		/* fd which is set to PPP discipline */
 static int rtm_seq;
+static int rtableid;

 static int restore_term;	/* 1 => we've munged the terminal */
 static struct termios inittermios; /* Initial TTY termios */
@@ -218,7 +219,9 @@ ppp_available()
 	return 1;		/* can't tell */

     strlcpy(ifr.ifr_name, "ppp0", sizeof(ifr.ifr_name));
-    ok = ioctl(s, SIOCGIFFLAGS, (caddr_t) &ifr) == 0;
+    ok = ioctl(s, SIOCGIFRDOMAIN, (caddr_t) &ifr) == 0;
+    if (ok)
+	rtableid = ifr.ifr_rdomainid;
     close(s);

     no_ppp_msg = "\
@@ -1115,6 +1118,7 @@ dodefaultroute(g, cmd)
     memset(&rtmsg, 0, sizeof(rtmsg));
     rtmsg.hdr.rtm_type = cmd == 's'? RTM_ADD: RTM_DELETE;
     rtmsg.hdr.rtm_flags = RTF_UP | RTF_GATEWAY;
+    rtmsg.hdr.rtm_tableid = rtableid;
     rtmsg.hdr.rtm_version = RTM_VERSION;
     rtmsg.hdr.rtm_seq = ++rtm_seq;
     rtmsg.hdr.rtm_addrs = RTA_DST | RTA_GATEWAY | RTA_NETMASK;