Fix IP_SENDSRCADDR semantics.
* To use this option with a UDP socket, it must be bound to a local port,
and INADDR_ANY, to disallow possible collisions with existing udp inpcbs
bound to the same port on other interfaces at send time.
* If the socket is bound to INADDR_ANY, specifying IP_SENDSRCADDR with
INADDR_ANY will be rejected as it is ambiguous.
* If the socket is bound to an address other than INADDR_ANY, specifying
IP_SENDSRCADDR with INADDR_ANY will be disallowed by in_pcbbind_setup().
Reviewed by: silence on -net
Tested with: src/tools/regression/netinet/ipbroadcast
d013c6882d