<sys/cdefs.h>: Simplify the _Noreturn compatibility macro.

Our g++ doesn't support [[noreturn]] yet. We might use [[gnu::noreturn]]
here for now, but thinking about it, simpler is better, so just make it
__dead2 for anything that doesn't claim to be C11.

Buildworld-breakage-originally-pointed-out-by: sephe
Merge branch 'master' into gsocdrm_jfree
udp: Fix IP source address setting for multicast address bound socket (2)

Source address should _not_ be set to INADDR_ANY in udp_output(), which
will make udp_output() calculate wrong pseudo header checksum.  Instead
of letting IP to select the source address, we extend in_pcbladdr() to
always locate the laddr, and use this function to find the source address
in udp_output().
ip/udp: Make input source address and source port saving MPSAFE

Change the global variable udp_in, udp_in6 and udp_ip6 into stack variable
and passing them around.  This should unbreak applications which depend
on received datagrams' source address or source port.
ifq: Expand IF queue operation macros

This prepares for the byte based queue limit and "rough priority queues"
netgraph7/source: ifnet.if_snd is not ifqueue
802.11: vap's if_snd is _not_ ALTQ ready
netgraph7/iface: Second parameter to ifq_dequeue should be NULL
polling: Update manpage
Merge branch 'master' into gsocdrm_github