Update perlguts for NULL in AVs
perlguts: consistent spaces after dots

It was inconsistent throughout.
perlguts: Note that SvUTF8 is meaningful only after SvPV
Update perlguts for the magic flags changes in 5.18
util.c:my_pclose: Use NULL in PL_fdpid

PL_fdpid is an AV, and as of ce0d59f AVs use NULL for nonexist-
ent elements.

Without using NULL for deleted elements of PL_fdpid, we end up with
pipes appearing to be open after they have actually been closed.

I don’t know how to write a test for this, but it makes
Proc::ParallelLoop pass its tests.