head(1): validate all line count arguments

head(1): validate all line count arguments

We need to validate all line count option arguments, not just the last
such argument found.

While here, switch to the default strtonum(3) error message format:

	<argument-name> is <errstr>: <argument-string>

Thread: https://marc.info/?l=openbsd-tech&m=163383023212104&w=2

ok millert@