Perl interface to POSIX.1-2008

Changes for 0.20 - 2023-07-14

• t/02_fs.t: Added work-around for bugs in CORE::stat().
• Pimped abs().
• Fixed nice() error handling.
• Added PRIO_*, FE_*, RWF_*, SEEK_DATA and SEEK_HOLE constants.
• Added some non-standard open() flags.
• Added preadv2(), pwritev2().
• Added remquo().
• Added feclearexcept(), feraiseexcept(), fetestexcept().
• Added isgreaterequal(), isless(), islessequal(), islessgreater(), isunordered().
• Added fenv_h export tag.
• POD improvements.