Cleaning Feta flags code (issue4236043)

[lemniskata . bernoullego]

This is a preparation for changes in the flag code that will be made
later (shortened flags affair).
This patch makes flags code more uniform and easier to maintain.
There is no actual change to the font itself, only to the way it is
coded.
(You may check this by running
  mf '\mode:=proof; input feta-noteheads20'
  gftodvi feta-noteheads20.2602gf
from /mf directory and comparing dvi files before and after a patch.)

cheers,
Janek

http://codereview.appspot.com/4236043/