Auto merge of #3224 - stevenengler:wait-for-one, r=JohnTitor

Add `MSG_WAITFORONE` to freebsd and openbsd

Adds `MSG_WAITFORONE` to freebsd, openbsd, and ~illumos~, as requested in nix-rust/nix#2014.

I got the values from:

- freebsd: http://fxr.watson.org/fxr/source/sys/socket.h#L473
- openbsd: https://github.com/openbsd/src/blob/2852e11abfc574a216ce741308fb0c6968d9617a/sys/sys/socket.h#L512
- illumos: https://github.com/illumos/illumos-gate/blob/717646f7112314de3f464bc0b75f034f009c861e/usr/src/boot/sys/sys/socket.h#L434

This flag is also supposedly supported on [solaris](https://docs.oracle.com/cd/E88353_01/html/E37843/recvmmsg-3c.html), but I don't know how to find the value. If anyone knows what it is, I'll add it to the PR.

libc-test/semver/freebsd.txt

@@ -785,6 +785,7 @@ MSG_NBIO
 MSG_NOERROR
 MSG_NOSIGNAL
 MSG_NOTIFICATION
+MSG_WAITFORONE
 NANOSECOND
 NETGRAPHDISC
 NET_RT_DUMP

libc-test/semver/openbsd.txt

@@ -527,6 +527,7 @@ MSG_CMSG_CLOEXEC
 MSG_DONTWAIT
 MSG_MCAST
 MSG_NOSIGNAL
+MSG_WAITFORONE
 MNT_LAZY
 MNT_NOWAIT
 MNT_WAIT

src/unix/bsd/freebsdlike/freebsd/mod.rs

@@ -3696,6 +3696,7 @@ pub const MSG_NBIO: ::c_int = 0x00004000;
 pub const MSG_COMPAT: ::c_int = 0x00008000;
 pub const MSG_CMSG_CLOEXEC: ::c_int = 0x00040000;
 pub const MSG_NOSIGNAL: ::c_int = 0x20000;
+pub const MSG_WAITFORONE: ::c_int = 0x00080000;
 
 // utmpx entry types
 pub const EMPTY: ::c_short = 0;

src/unix/bsd/netbsdlike/openbsd/mod.rs

@@ -1073,6 +1073,8 @@ pub const IP_RECVIF: ::c_int = 30;
 pub const TCP_MD5SIG: ::c_int = 0x04;
 pub const TCP_NOPUSH: ::c_int = 0x10;
 
+pub const MSG_WAITFORONE: ::c_int = 0x1000;
+
 pub const AF_ECMA: ::c_int = 8;
 pub const AF_ROUTE: ::c_int = 17;
 pub const AF_ENCAP: ::c_int = 28;