unix sockets: Linux is fine on any architecture

No need to limit ourselves to x86.
Regardless of architecture, sun_path is always 108 bytes long on Linux.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
unix sockets: reworked to support abstract namespace

Rewritten to read sligthly easier while adding support for abstract
namespaces on Linux.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
tests, doc edits, and minor changes for unix sockets