This reverts commit 262352f8bb, reversing
changes made to 312691376a due to a bug
whereby `weave env` returns a unix domain socket with an incorrect path.
This change will be superceeded by #1694.
This lets you use -H unix://*.sock outside /var/run/weave. Using a
socket directly in /var/run (i.e. /var/run/foo.sock), will reintroduce
the bug with namespaces not being GCed, so that is a warning.