Fix check_swap returning OK on & thresholds and no swap

Return 0% free instead of 100% free when total swap is 0
Typo
Add strcase gnulib module
Move negate' translate_state() to utils_base.h mp_translate_state()

Also use strcasecmp imported from gnulib for simplicity
check_swap: add supports for a configurable state when there is no swap

Check_swap used to allow no swap when thresholds were only specified in
percent. This is no longer the case and the state now must be specified
explicitly. The default is to always return CRITICAL when the swap is
absent regardless of thresholds.
AUpdate .gitignore for new gnulib module
Update check_swap usage