Instead, we create the socket in a temporary directory - TMPDIR if it is
not too long, or else /tmp. The directory is made via `mkdtemp` with the
template `urbit-XXXXXX`. The socket is located there (getting around the
length limit) and `$pier/.urb/conn.sock` is symlinked to it.

Replaces calls to uv_strerror with strerror for system calls; the former
does not print system call errors properly.

The pier socket link is predictable; no information is conveyed by
printing it.

Now uv_close is never needed as part of failure cleanup. N.B. this can
be moved before the close to keep the cleanup logic stacked.

Also handles 0-length TMPDIR.