bug#53125: Missing abstract unix socket support

Currently, It's not valid to create abstract unix socket in Guile via
`connect` procedure, because it rejects string with leading NUL.

Abstract unix socket is widely used in D-Bus. Missing support for it
hurts the ability of Guile in D-Bus programming.

I see a patch at
https://lists.gnu.org/archive/html/guile-devel/2021-03/msg00038.html

But looks that nobody cares it. Would someone can help review and merge it? 
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Retrieve my PGP public key:

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Zihao

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