[perl #121676] Fix memory leak in backticks and system

Introduced by perl #113536. It doesn't actually leak on Windows XP due to
differences in the implementation of GetEnvironmentStringsW() compared to
later OSes, but the missing FreeEnvironmentStrings() was technically wrong
anyway, and does now bite. Thanks for Daniel Dragan for finding this.
perldelta for #121676
Add links for perl bug tickets in previous commit
Fix commit 90674eaa88

Spotted by Daniel Dragan. We use GetEnvironmentStringsW() (deliberately so:
see commit 4f46e52b00), so we must use FreeEnvironmentStringsW() to match.
document the API signature change for Perl_regexec_flags()

and its related plugin structure pointer

inspired by https://rt.perl.org/Ticket/Display.html?id=121868 which was
also broken by ea3daa5d5f60389b105c00d4f7d05b5c2f2155f2
POSIX.pod: Add clarification for strtod();
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.
Add notice for removal of find2perl, s2p and a2p
Note that the minimum supported gcc (on Windows) is 3.4.5

Earlier versions (from www.mingw.org, at least) will build perl, but have
a linker bug that causes various tests to fail due to problems with
SDBM_File.dll. See perl #121936 for details.
Document known test failure in www.mingw.org builds of gcc-4.8.1 (at least)

This came up in perl #121643, but I forgot to add a note of this Known
Problem.
Update PathTools version in Porting