add 'head' field to PerlIOl struct

add 'head' field to PerlIOl struct

This allows any layer to find the top of the layer stack,
or more specifically, the entry in PL_perlio that points to
the top.

Needed for the next commit, which will implement a reference counting
scheme.

There's currently a bug in MakeMaker which causes several extensions to
miss the dependency on perliol.h having changed, so this commit includes
a gratuitous whitespace change to perl.h to hopefully force recompilation.