first step to documenting the Internals namespace

first step to documenting the Internals namespace

After discussion, we have decided to document Internals. This is a
first step doing so, with lib/Internals.pod. At the same time I
updated sv.h that there were equivalents for SvREFCNT and SvREADONLY
in the Internals namespace, this required adding docs for SvREADONLY.

I also tweaked the test name in Porting/Maintainers.pm
so that it pointed at Porting/Maintainers.pl to make it more obvious
how to specify a maintainer for a new file.