lint: rename snprintb 'directives' to 'conversions'

This aligns the terminology with the snprintf function.

lint: fix warning about "empty" single-letter snprintb descriptions

lint: check for unreachable bits and fields in snprintb formats

While here, clean up a few existing checks.

lint: test format strings from snprintb calls

The functions snprintb and snprintb_m are specific to NetBSD, and their
format strings are tricky to get correct. Provide some assistance in
catching the most common mistakes.