getsockopt.2: Add SO_SPLICE source socket read and socket close behavior

[kbowling]

Summary

Add select(2), poll(2), or kqueue(2) source socket semantics and close behavior.

Reported by: Udip Pant <[e-mail]>
Obtained from: OpenBSD

[kbowling]

@markj I think this behavior is identical to OpenBSD so adding part of their man doc.

lib/libsys/getsockopt.2

598

https://cgit.freebsd.org/src/tree/sys/kern/uipc_socket.c#n4444

600

kqueue filt_soread: https://cgit.freebsd.org/src/tree/sys/kern/uipc_socket.c#n4558
Test: https://cgit.freebsd.org/src/tree/tests/sys/kern/socket_splice.c#n469

[ziaee]

Ok from manpages@, thank you!

[markj]

lib/libsys/getsockopt.2

602

A bit pedantic, but really it indicates that the splice has terminated _and_ that at least one byte is available for reading. Unsplicing itself doesn't cause an event to be raised.