update CoreList for 5.20.0
perldelta: generate acknowledgements for 5.20.0
perldelta: update modules sections for 5.20.0
remove the perl519*delta documents
update perlhist for v5.20.0
patchlevel: we are now v5.20.0-RC1
PATCH: [perl #121630]: perlpod clarification
perlfunc: Should chdir("/") immed after chroot

Inspired by Coverity perl5 CID 28931: Insecure chroot: (CHROOT)
chroot_call.

The Perl chroot() is a thin wrapper around the system call, so the
chdir("/") should not go there.  But adding a note about the chdir()
being a good idea to perlfunc/chroot.

The committer reworded the inserted text for grammar and readability,
and the commit message to conform to git standards, and is pushing the
commit with the concurrence of the author.
perldelta: update known problems
perldelta: Note patch fixed a bunch of tickets

Unbeknownst to the authors, several other tickets got fixed.