Correct month, quarter (3 months), and 4 year RRD graphs length.

Longest possible month is 31 days, not 32.
Longest possible quarter (3 months) is 92 days (30+31+31), not 93 (31+31+31).
4 Years is 1461 days (365 x 4 + 1), not 1464 (366 x 4).  Except about once every century or so when leap year is skipped 4 years is then 1460 days.  But that is not going to happen again for a very long time.  Unfortunately pfSense, nor anyone alive today will probably still be alive by then.
Merge pull request #701 from N0YB/RRD_Graphs_Size

Correct month, quarter (3 months), and 4 year RRD graphs length.