[lib] Switch Fletcher checksum back to old ospfd version

[lib] Switch Fletcher checksum back to old ospfd version

* lib/checksum.c: (fletcher_checksum) Switch the second phase of the checksum
  back to the old ospfd logic.

  The isisd-derived version:

  a) is very hard to follow
  b) had some kind of subtle bug that caused it be wrong when c0=0 and c1=254
     (potentially fixable by doing the mods before adjusting x and y)

  Additionally:

  - explicitely cast expressions using non-internal variables to int, to ensure
    the result is signed.
  - defensively change the length argument to 'size_t', to ensure the code
    works with that argument being unsigned..

  Thanks to Joakim Tjernlund for the investigative work into this bug.

* tests/test-checksum.c: new file to exercise the checksum code.