Trust a few system headers to stand on their own

Trust a few system headers to stand on their own

At some point in the past some headers (net/if.h on some BSDs in 2009,
netinet/tcp.h on IRIX in 2000, sys/ucred.h in 2013 on then older openbsd),
only compiled if other heades were included first, complicating configure
tests. More recent tests indicate that that's not required anymore.

Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/20220807012914.ydz73yte6j3coulo@awork3.anarazel.de