xenstore: new XS_OPEN_SOCKETONLY flag; honour "-s" on tools' cmdline

xenstore: new XS_OPEN_SOCKETONLY flag; honour "-s" on tools' cmdline

The "-s" option to xenstore-ls is used by the xencommons startup
script to check whether xenstored is already running, before starting
it.

Since 22498:a40455ae9ad3, "-s" has been a no-op, and libxenstore will
always fall back to using xenbus.  The combined effect is that the
xencommons script deadlocks: xenstore-ls hangs waiting for xenstored,
which isn't started by xencommons because xencommons is waiting for
xenstore-ls.

In this patch, we:
 * Introduce a new XS_OPEN_SOCKETONLY flag which disables the
   fallback behaviour;
 * Make the xenstore command line tools use the new xs_open call
   rather than the old, deprecated xs_open_* calls (which are
   now identical).
 * Plumb the xenstore command line tools "-s" option to set the
   XS_OPEN_SOCKETONLY flag.
 * Change the type of the XS_OPEN_* flags so that they naturally have
   type unsigned long.

The "-s" option to xenstore-ls et al, and the XS_OPEN_SOCKETONLY flag,
are intended for use by toolstack infrastructure and should not
normally be used by higher-level code.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>