linux-user: Implement sendmmsg syscall

linux-user: Implement sendmmsg syscall

Glibc when built for newer kernels assumes that the sendmmsg syscall is
available. Without it, dns resolution simply fails to work.

Wrap the syscall with existing infrastructure so that we don't have a host
dependency on sendmmsg.

To avoid locking the same area of guest memory twice (which will break if
DEBUG_REMAP is defined) we pull the lock/unlock part of do_sendrecvmsg()
out into its own function so the actual implementation can be shared.

Signed-off-by: Alexander Graf <agraf@suse.de>
[PMM: add recvmmsg support;
 handle errors (which also implies support for non-blocking operations);
 cap the vector length as the kernel implementation does;
 don't lock guest memory twice;
 support MSG_WAITFORONE flag]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>