Support AF_UNIX sockets

Based on previous work resultant from [1], which seem to first have been implemented in rust's libstd but then reverted (in commit 0498da9), this adds support for AF_UNIX to relibc. It follows the same approach of using the "chan:" scheme, backed by ipcd (another set of patches is submitted for ipcd, to back this change).

It adds SOCK_STREAM support, but not SOCK_DGRAM, which is planned to be added later.

[1] https://www.redox-os.org/news/rsoc-porting-tokio-1/