client: search likely paths for the xenstore socket

We look for:

  XENSTORED_PATH (if set)
  the Unix domain socket
  /proc/xen/xenbus
  /dev/xen/xenstore

Note we have to open the /proc file in a special way because it
behaves more like a pipe or socket but isn't manifested as a pipe
or socket in stat(2).

Signed-off-by: David Scott <dave.scott@citrix.com>