[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.

(cherry picked from commit 90674eaa88a3b2f52d8ac1b835c3505b25ff3f00)
Fix commit 2358a1918b

Spotted by Daniel Dragan. We use GetEnvironmentStringsW() (deliberately so:
see commit 4f46e52b00), so we must use FreeEnvironmentStringsW() to match.

(cherry picked from commit a6abe943fc6abe60b49c81ccb55b1e99f8993c8d)
perldelta for #121676

(cherry picked from commit 58627cdf06c592b676923193c3e08c74f75129a3)
Add links for perl bug tickets in previous commit

(cherry picked from commit 47e6d4dbb87eb14b29f00df5304f5d8bf44236ac)