util-lib: when parsing time values, accept "M" as short for "month"

util-lib: when parsing time values, accept "M" as short for "month"

nginx defines an uppercase "M" that way (in contrast to the lowercase
"m" for "minute"), and it sounds like an OK logic to follow, so that we
understand a true superset of time values nginx understands.

http://nginx.org/en/docs/syntax.html