Fix configure probe for sys/ucred.h.

The configure script's test for <sys/ucred.h> did not work on OpenBSD,
because on that platform <sys/param.h> has to be included first.
As a result, socket peer authentication was disabled on that platform.
Problem introduced in commit be4585b1c27ac5dbdd0d61740d18f7ad9a00e268.

Andres Freund, slightly simplified by me.