lint: KNF

branches: 1.3.2;
lint: warn about escaped snprintb directive

Repurpose message 362, as the previous version was redundant since null
bytes in old-style formats are already covered by message 371 (bit
position out of range) and 377 (redundant '\0' at the end).

lint: check snprintb formats for overlapping bits and fields

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.