[krbdev.mit.edu #8350] Add IP_RECVDSTADDR/IP_SENDSRCADDR support to net-server.c

[Greg Hudson via RT]

Most common platforms now support IP_PKTINFO (Linux, Solaris, NetBSD, OS 
X, Windows), but FreeBSD only supports a variant using IP_RECVDSTADDR and 
IP_SENDSRCADDR.  We should support this variant since we have decided to 
remove the bind-to-all-interfaces workaround.

OpenBSD (and I think old versions of OS X) define IP_RECVDSTADDR but not 
IP_SENDSRCADDR.  It's conceivable that IP_RECVDSTADDR can be used to 
specify a UDP source address (IP_SENDSRCADDR has the same integer value 
as IP_RECVDSTADDR on FreeBSD), but we would need to test it.  If that 
doesn't work, OpenBSD by itself isn't sufficient reason to keep 
supporting the workaround of binding to all interfaces.  But if it does 
work it would be nice to support it.