Fix an inconsistency in socket option propagation on accept(). Propagate the SS_NBIO flag from the parent socket to the child socket during an accept() operation.
Fix an inconsistency in socket option propagation on accept(). Propagate
the SS_NBIO flag from the parent socket to the child socket during an
accept() operation.

The file descriptor O_NONBLOCK flag would have been propagated already
by the fflag assignment, and therefore would have been inconsistent
with the underlying socket's so_state member.

This makes accept() more closely adhere to the API contract we effectively
outline in the manual page. Note also that Linux continues to differ here;
O_NONBLOCK is not propagated. The other BSDs do propagate the flag, as
does Solaris. The Single UNIX Specification does not offer specific
advice on this issue.

PR:		kern/45733
Requested by:	Jayanth Vijayaraghavan
Reviewed by:	rwatson

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