pppd: Don't crash if NULL pointer passed to vslprintf for %q or %v

This is to avoid crashes like that reported in
https://bugs.launchpad.net/ubuntu/+source/ppp/+bug/1244714

Signed-off-by: Paul Mackerras <paulus@samba.org>
pppd: Accept IPCP ConfAck packets containing MS-WINS options

Since last week I'm seeing IPCP negotiations going like this (and
eventually failing) when connecting to my ISP:

Jul 11 20:03:25 * pppd[4833]: sent [IPCP ConfReq id=0x2 <addr 0.0.0.0> <ms-dns1
0.0.0.0> <ms-dns2 0.0.0.0>]
Jul 11 20:03:26 * pppd[4833]: sent [IPCP ConfReq id=0x2 <addr 0.0.0.0> <ms-dns1
0.0.0.0> <ms-dns2 0.0.0.0>]
Jul 11 20:03:26 * pppd[4833]: rcvd [IPCP ConfNak id=0x2 <addr 10.167.246.198>
<ms-dns1 213.162.69.1> <ms-dns2 213.162.69.169> <ms-wins 124.6.168.55> <ms-wins
17.17.17.17>]
Jul 11 20:03:26 * pppd[4833]: sent [IPCP ConfReq id=0x3 <addr 10.167.246.198>
<ms-dns1 213.162.69.1> <ms-dns2 213.162.69.169> <ms-wins 124.6.168.55> <ms-wins
17.17.17.17>]
Jul 11 20:03:26 * pppd[4833]: rcvd [IPCP ConfAck id=0x3 <addr 10.167.246.198>
<ms-dns1 213.162.69.1> <ms-dns2 213.162.69.169> <ms-wins 124.6.168.55> <ms-wins
17.17.17.17>]
Jul 11 20:03:27 * pppd[4833]: sent [IPCP ConfReq id=0x3 <addr 10.167.246.198>
<ms-dns1 213.162.69.1> <ms-dns2 213.162.69.169> <ms-wins 124.6.168.55> <ms-wins
17.17.17.17>]
...

with the last two lines repeating until the IPCP error limit is
reached. As you can see, the peer added two extra fields in the
ConfNak reply. This is allowed, and indeed the following sent
ConfReq packet reflects this. However, when the ConfAck packet
is received, pppd discards it as invalid, because of the ms-wins
fields.

This fixes it.

Signed-off-by: Paul Mackerras <paulus@samba.org>
config: Update Solaris compiler options and enable CHAPMS and IPV6

As suggested by James Carlson.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Update README and patchlevel for 2.4.6 release

Signed-off-by: Paul Mackerras <paulus@samba.org>