Add PERL_UNUSED_RESULT() and use that instead of the V_Gconvert().

(cherry picked from commit ffea512f3e5e645945e337a9aa70a304cd695df3)
Move the PERL_GCC_BRACE_GROUPS_FORBIDDEN earlier.

The PERL_UNUSED_RESULT needs it to avoid using the brace groups
when going -pedantic.

(cherry picked from commit 24e7ff4ea7e35b5aed3b22dd00848a50f81fad4b)
Use PERL_UNUSED_RESULT.

(1) Enhance its description.
(2) Simplify it: define only if has warn_unused_result.
(3) Make it use STMT_START { ... } STMT_END to be less GNU-extensiony.
(4) Redo 04783dc7 ("fix 'ignoring return value' compiler warnings") with it.

(cherry picked from commit b469f1e0fc5f0ac882161e627a1255ee11e67c37)
perlop: Update text to reflect code changes

The warning message is no longer misleading, so no need to point out
anything about it.

(cherry picked from commit 7ea906bb370ef79b0aee20cbb8768c72eb0d22c6)
perlop: Nits

(cherry picked from commit c3e9d7a98ad8340994f5f9d9ddf6c792f0875b81)
POSIX.pod: Document correct redirect for 'atexit'.

For: RT #122412
(cherry picked from commit ce5b0b849c4a3e4d77dc60096ae4170609a81644)
perldelta - Update list of upgraded modules