Reverted month_length() back to using the 'knuckles' algorithm. Easier to understand, and a couple words less FLASH.

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2358 1084037a-409f-4a99-8160-1a4b3577d4c1
Fixed a warning due to unused variable.

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2359 1084037a-409f-4a99-8160-1a4b3577d4c1