network: Migrate SIGPIPE generation into the socket module.

This removes the burden of determining whether to and then
actually sending SIGPIPE from the protocol modules, meaning
the MSG_NOSIGNAL flag can now be implemented entirely in
the socket module and not even passed further down the chain.

Change-Id: I9ba976c4aff60d533cb4b390bbba1560c0de423f
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7124
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

protocols/unix: Implement MSG_NOSIGNAL.

Same logic as in the TCP protocol module.

This fixes a regression from hrev57383: curl now works
again. (Perhaps it shouldn't be called a "regression",
though, but instead an uncovered bug / missing feature.)

Fixes #18666.

protocols/unix: Return EOPNOTSUPP when unhandled flags are specified.

Fixes the "bug" part of #18653 (however the flags still need to
actually be implemented.)

Change-Id: Icd296af8b409416317ba3d02735504729949fd08
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7109
Reviewed-by: Niels Sascha Reedijk <niels.reedijk@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

unix: respect MSG_DONTWAIT on recvmsg

fix #18548

Change-Id: I33f502c2a376be6dbdf913f9613aab9d4c5a3644
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6802
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

unix: respect MSG_DONTWAIT on sendmsg()

fix bug #18539

Change-Id: Id21362028287d1cbdac469226e6b52f4547a276f
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6796
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

unix: Report connected status to socket module

Report connected status to the socket module. Otherwise, operations
like `getpeername` would not work correctly.

Fixes #18534.

Change-Id: I99d047f0d7b4d442cc2b3ea9222b0d89d216c1ae
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6719
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

unix: Implement datagram sockets

Implement `SOCK_DGRAM` sockets for `AF_UNIX` family.

Change-Id: If3d6f408a7d881635ccf04b080391905fdc94b13
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6617
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>