Fix Issue #308

[pangyikhei]

Fixes issue #308 by checking if there is a value for the L case.

Added a before_last_day_of_month resource string.

[pangyikhei]

I used Google Translate to translate "before the last day of the month".
Technically the word order in Chinese when combined with "X days" is grammatically incorrect (the days should come after).
I'm not sure about the other non-English languages.

Actually, I'm thinking I should create multiple resource strings so the grammar is better than using English as a reference when building the string.

[pangyikhei]

Replaced before_last_day_of_month with two resource strings: day_before_last_day_of_month and days_before_last_day_of_month so it is possibly more grammatically correct in more languages.

[jmrozanec]

@pangyikhei merged! Thank you for contributing to cron-utils! 😄