PG versus libxml2 2.12.x

Buildfarm member caiman has been failing build for a couple weeks now.
The reason turns out to be that recent libxml2 has decided to throw
a "const" into the signature required for custom error handlers.
(API compatibility?  What's that?)

I don't mind adopting the "const" --- it's a good idea in isolation.
The trouble is in fixing our code to work with both old and new
libxml2 versions.  We could thrash around with a configure test or
something, but I think the most expedient answer is just to insert
some explicit casts, as shown in the attached.  It's possible though
that some compilers will throw a cast-away-const warning.  I'm
not seeing any, but ...

Also, I'm seeing a deprecation warning in contrib/xml2/xpath.c
for

	xmlLoadExtDtdDefaultValue = 1;

I'm not sure why that's still there, given that we disabled external
DTD access ages ago.  I propose we just remove it.

In short, I suggest the attached.

			regards, tom lane