[test] Update Pod::Simple to 3.28

fixes missing whiteline_handler method.
add =encoding handling.

GH #1089
[codingstd] fix various pod syntax errors

These errors arose with the updated Pod::Simple/Checker, added with GH #1089
[codingstd] fix GH #1090 5.8 crashes with DEBUGGING

3 =~ ///gsx substs cause perl5.8 with to crash at
t/codingstd/c_enum.t,
t/codingstd/c_macro_args.t and
t/codingstd/c_cppcomments.t when DEBUGGING is enabled.

Skip those tests then.