rpc: New rpc_unix transport based on Unix socket

[ueno]

This is split from #15 and basically identical to NetworkManager@fbd884b. The only differences are:

• rpc_unix_connect() does not return CKR_DEVICE_ERROR, so make rpc_C_Initialize() happy
• use '/' as the prefix, instead of 'unix:path=', as suggested in https://lists.freedesktop.org/archives/p11-glue/2014-November/000516.html

[lkundrak]

While I agree with the mailing list post that '/' would be better than '@', but I still maintain that 'unix:path=' would be better (as mentioned in the standalone server pull request). Hopefully I'm not bikeshedding too much.

The D-Bus transport name specifiers are already familiar to users and would make it easy to add new transports that may come useful in future, such as unix:tcp or unix:runtime (or even add unixexec: alias for | for no reason at all).

[ueno]

Yes, having a readable prefix for transport names sounds reasonable. However, for Unix domain sockets, IMO only "path" is relevant (in the D-Bus spec, "tmpdir" and "runtime" are shortcut for servers, and "abstract" doesn't really make sense these days; also "tcp" is not really Unix specific). So I have changed it from '/' to "unix=/".

Also added a standalone test case.

[ueno]

I ended up with "unix:path=" for consistency with DBUS_*_ADDRESS.