We need this so that the tor_api user can specify some arguments,
while the tor_api implementation adds others.

This implementation detail should not be visible to tor_api users.

This change also makes tor_ersatz_socketpair() follow the same
interface as socketpair() rather than tor_socketpair(), so it now
needs to be wrapped in the same code as socketpair() does.

Also refactor some annoying code in tor_ersatz_socketpair.

I'm doing this because I want to make it a lower-level function
again, so that we can use it without linking in the rest of the
universe.

Part of 26979.

Closes 26979.

To avoid future duplicates.

Part of 26979.

Part of 26979.

Fixes bug 26986; bugfix on master.

Part of 26979.

That's what git does for tor.

Part of 26979.

Part of 26979.