Xpra: Ticket #1525: port peercred to macos, bsd, etc

Follow up from #1524. We hardcode this linux value:

SO_PEERCRED = 17

This needs to be moved to platform code.

Sat, 23 Jan 2021 05:26:42 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1525