unix: Add support for atomically setting the socket mode

[markj]

Summary

With this patch, it is possible to call fchmod() on a unix socket prior
to binding it to the filesystem namespace, so that the mode is set
atomically. Without this, one has to call chmod() after bind(), leaving
a window where threads can connect to the socket with the default mode.
After bind(), fchmod() reverts to failing with EINVAL.

I believe this is compatible with Linux.

I did not modify the behaviour of fstat(), i.e., it continues to return
the mode as set by soo_stat().

PR: 282393

[markj]

sys/_types.h is sufficient.

[markj]

Fix an inverted test.

[markj]

Update man pages, add some tests.