unix: fix host UDS recvmsg results

[russellromney]

Summary

Fix host Unix socket recvmsg passthrough.

The host UDS path already connects to bind-mounted host socket inodes. The receive side was returning the wrong result to the guest.

This change:

• translates the Quark hostfd to the real OS fd before libc::recvmsg
• returns the actual recvmsg byte count
• returns errno as an error, not as a positive byte count
• uses the host UDS recv wrapper on blocking retries
• adds a static C regression client

Fixes #1369.

Test

Original bind-mounted UDS probe:

host process -> OK
runc         -> OK
runsc        -> connect-failed errno=111
quark rw     -> OK
quark ro     -> OK

Regression client under Quark:

host_uds_recv: 8 iterations ok
mode=rw ok
host_uds_recv: 8 iterations ok
mode=ro ok

[yaoxin-jing]

looks good to me