unix: add Linux-compatible abstract sockets

[glebius]

Summary

Abstract UNIX sockets live in a namespace that is not connected to the
file system. The namespace is global (per jail) and no permission checks
are applied. However applications may use LOCAL_PEERCRED and LOCAL_CREDS
mechanisms to learn about their peer.

A request for an abstract namespace is marked with leading NUL byte in
sun_path on bind(2) or connect(2).

In collaboration with: Ricardo Branco <[e-mail]>
In collaboration with: bapt

[rbranco_suse.com]

This diff doesn't cleanly apply on current main (b06db8c9). There are some rejects for sys/kern/uipc_usrreq.c

I adapted it and tested in this branch:
https://github.com/ricardobranco777/freebsd-src/tree/abstractx

The old & new tests run fine.

[glebius]

It doesn't apply cause it depends on D60968.

[rbranco_suse.com]

In D60969#1398512, @glebius wrote:

It doesn't apply cause it depends on D60968.

Applied both and all relevant tests pass.

Thanks.

[bapt]

tested again, for the rcd(8) uses cases it works perfectly.