auth: Clean up and document functions in netrestrict.c

Clean up and document parseNetRestrictFile_int and ParseNetInfoFile_int
in preparation for some future changes.

Change-Id: I3c8f1823ba1e042fb6ae68c6f0aba58128ef5b81
Reviewed-on: http://gerrit.openafs.org/11312
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>