Fall back to unsigned int, not int, for socklen_t.

Fall back to unsigned int, not int, for socklen_t.

It's a coin toss which of these is a better default assumption.
However, of the machines we have in the buildfarm, the only ones
relying on the fallback socklen_t definition are ancient HPUX,
and on that platform unsigned int is the right choice.  Minor
tweak to ee3a1a5b6.

Discussion: https://postgr.es/m/1440792.1636558888@sss.pgh.pa.us