The host accepts N simultaneous connections and writes 1 byte to them
each. Clients connect and read 1 byte in order to obtain a thread token.

std.Thread.Pool now lazily spawns threads only when the work queue is
non-empty. I think that was a bad idea and will revert it shortly.

There is now a std.zig.initThreadPool wrapper that deals with:
* Resolving a zig cache directory into a UNIX domain socket address.
* Creating the "tmp" directory in .zig-cache but only if the listen
  failed due to ENOENT.
* Deciding to connect to an existing jobserver, or become the host for
  child processes.