Extend strictness for qr/(?[ \N{} ])/

This recently added regex syntax imposes stricter rules on parsing than
normal.   However, this did not include parsing \N{} constructs that
occur within it.  This commit does that, making fatal the warnings that
come from \N{}

I will add to perldiag the newly added messages along with the others
for (?[ ]) before 5.18 ships