agetty.c: further scrubbing

agetty.c: further scrubbing

In reply to two requests.

http://permalink.gmane.org/gmane.linux.utilities.util-linux-ng/3990
http://permalink.gmane.org/gmane.linux.utilities.util-linux-ng/3999

The scubbing consists;

o Old school introduction back to the top of the source file.
o Indentation changes.
o Removal of void casting.
o Removal of braces from single statements.
o Unified format for comments.
o Comparison with W. Venema's code from 1993 to clean few comment
  mysteries.
o Web referrals added to further explanation to near the code or
  comments which some could find difficult to understand why.
o Clean up of comments.
o FIXME items added where attention is needed.
o Make functions static.
o Functions with same argument types use same argument order.
o Get rid of classic K&R style function declarations.

Please notice that this patch _should not_ affect how program
works e.g. this has no actual code changes.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>