Add docs describing that generate_socket_name() returns a
pseudo-random _abstract_ socket name, without the leading ␀-char.

Add details on what _abstract_ Unix domain sockets are, and that
they're used when the input path is NULL.

These input variables are not changed, so we should make them const.

Combines variable definition and initialization, since we aren't using
an old C89 compiler.

Additionally, I've replaced all of the sockaddr_un initialisations
with portable C default initializers, as memset() is explicitly
not recommended as it isn't portable,
see [manpage for netinet_in.h under APPLICATION USAGE][1]

[1]: https://manpages.ubuntu.com/manpages/jammy/en/man7/netinet_in.h.7posix.html

Instead of creating our own abstract address, using 8 random hex chars,
we can use Linux's autobind feature to create a new **unused** abstract
address consisting of 5 random hex chars.

This is 3 less hex chars, however Linux's autobind feature ensures
that the a new abstract address is always picked, unlike our custom
generator, which may rarely create an abstract address that is already
being used.

See https://manpages.ubuntu.com/manpages/jammy/en/man7/unix.7.html for
documentation on the "autobind feature".

Creates a helper function that can be used to:
- close a unix domain socket, and
- unlink() the unix domain socket (if _pathname_ unix domain socket)

I've modified the `close()` function after every
create_domain_client(NULL) to instead call `close_domain_socket()`.

…ockets

Currently, creating a temporary unix domain socket using
`create_domain_client(NULL)` creates an abstract Unix domain
socket, which only works on Linux.

In order to support FreeBSD (and other Unix not-Linux platforms),
`create_domain_client(NULL)` can now fallback to creating
a new _pathname_ unix domain socket using `mkdtemp()`.

There's a very tiny chance that we'll misidentify a folder
as a mkdtemp folder created by `create_tmp_domain_socket_path()`.

For this case, I've added a log statement. It's only a log_debug(),
since we expect that if this ever does misidentify a folder, rmdir()
will throw an ENOTEMPTY (dir not empty) error, which will print
an error log message.