tools/libs/store: drop read-only functionality

tools/libs/store: drop read-only functionality

Today it is possible to open the connection in read-only mode via
xs_daemon_open_readonly(). This is working only with Xenstore running
as a daemon in the same domain as the user. Additionally it doesn't
add any security as accessing the socket used for that functionality
requires the same privileges as the socket used for full Xenstore
access.

So just drop the read-only semantics in all cases, leaving the
interface existing only for compatibility reasons. This in turn
requires to just ignore the XS_OPEN_READONLY flag.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wl@xen.org>