Fix some configure header prereqs

Fix some configure header prereqs

On at least Solaris, the configure tests for netinet/if_ether.h and
security/pam_modules.h issued warnings because they existed but were
not compilable. Perform the tests with the prerequisite headers of
net/if.h and security/pam_appl.h, respectively, so autoconf will stop
yelling at us.

Change-Id: I05d637784954c10af468b6065acd78139fc45245
Reviewed-on: http://gerrit.openafs.org/4467
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>