Convert POSIX::sleep to an XS wrapper for PerlProc_sleep().

Convert POSIX::sleep to an XS wrapper for PerlProc_sleep().

Previously it was a Perl wrapper for CORE::sleep, converting CORE::sleep's
return value of elapsed time slept into the POSIX return value of seconds
remaining. However, that approach could sometimes return a negative result
if CORE::sleep had slept for more than a second longer than the requested
time.