Unix domain (IPC) sockets implementation

[Marcin Moskal]

As far as I know usocket does not provide implementation of Unix
domain sockets, nonetheless SBCL[1], CCL[2] and ECL[3] (and maybe
other) implement them. After doing some research I see that feature in
usocket is desired (see: usocket-devel question[4], another
usocket-devel question[5]), even partially implemented[6] by @tmccombs.

I would like to try to implement this feature without breaking the
current API. It could be done by assuming that the ``host'' parameter in
socket-connect function takes pathname -- indicating we mean the *Unix*
kind of socket. In this particular case we will ignore the ``port''
portion(or signal condition when it is not ``nil'').

I am looking for comments and advice.

[1] http://www.sbcl.org/manual/#Local-_0028Unix_0029-Domain-Sockets
[2] http://ccl.clozure.com/manual/chapter8.1.html
[3] https://gitlab.com/embeddable-common-lisp/ecl/blob/develop/contrib/sockets/sockets.lisp#L865
[4] https://mailman.common-lisp.net/pipermail/usocket-devel/2008-August/000198.html
[5] https://mailman.common-lisp.net/pipermail/usocket-devel/2009-February/000251.html
[6] https://github.com/usocket/usocket/compare/master...tmccombs:unix-sockets

--
Regards,
Marcin Moskal