Deprecate multiple "x" in "/xx"

Deprecate multiple "x" in "/xx"

 It is planned for a future Perl release to have /xx mean something
 different from just /x.  To prepare for this, this commit raises a
 deprecation warning if someone currently has this usage.  A grep of CPAN
 did not turn up any instances of this, but this is to be safe anyway.

 The added code is more general than actually needed, in case we want to
 do this for another flag.