adding strndup - Mailing list pgsql-hackers

I just proposed in
https://postgr.es/m/0191204143715.GA17312@alvherre.pgsql the addition of
strndup() to our src/port.

I think this should be pretty uncontroversial, but wanted to give a
heads-up outside that thread.  I attach the patch here for completeness.

-- 
Álvaro Herrera                                http://www.twitter.com/alvherre