misc cleanup in sysctl_ifmalist():

misc cleanup in sysctl_ifmalist():
 + remove a partly incorrect comment that i introduced in the last commit;
 + deal with the correct part of the above comment by cleaning up the
   updates of 'info' -- rti_addrs needd not to be updated,
   rti_info[RTAX_IFP] can be set once outside the loop.
While at it, correct a few misspelling of NULL as 0, but there are
way too many in this file, and i did not want to clutter the
important part of this commit.