domain, not a file descriptor.  Use 'domain' instead of the original 'd'
for this argument to match socket(2).

PR:		199491
Reported by:	sp55aa@qq.com
MFC after:	1 week

used "fd", while these used "d" and "filedes".

MFC after:	1 week
Approved by:	gjb
Approved by:	re (delphij)

This change allows creating file descriptors with close-on-exec set in some
situations. SOCK_CLOEXEC and SOCK_NONBLOCK can be OR'ed in socket() and
socketpair()'s type parameter, and MSG_CMSG_CLOEXEC to recvmsg() makes file
descriptors (SCM_RIGHTS) atomically close-on-exec.

The numerical values for SOCK_CLOEXEC and SOCK_NONBLOCK are as in NetBSD.
MSG_CMSG_CLOEXEC is the first free bit for MSG_*.

The SOCK_* flags are not passed to MAC because this may cause incorrect
failures and can be done later via fcntl() anyway. On the other hand, audit
is expected to cope with the new flags.

For MSG_CMSG_CLOEXEC, unp_externalize() is extended to take a flags
argument.

Reviewed by:	kib

clause.

# If I've done so improperly on a file, please let me know.

Stop calling system calls "function calls".

Use "The .Fn system call" a-la "The .Nm utility".

When referring to a non-BSD implementation in
the HISTORY section, call syscall a function,
to be safe.

Approved by:	re

Reviewed by:	ru