Conflicts: testcases/t/159-socketpaths.t
With this commit, i3 will now use either $XDG_RUNTIME_DIR/i3 (XDG_RUNTIME_DIR is only writable by the user, so this is not a problem) or a secure temporary location in /tmp, following the pattern /tmp/i3-<user>.XXXXXX
this will pave the way to just call BAIL_OUT and stop the currently running testfile if i3 died for whatever reason.
