[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