v2.4.1: Match less' behavior with piped stdin

Piped stdin, no args: Just page stdin

Piped stdin with args: Page args, ignore stdin. Except if one arg is
"-", then stdin will be paged in place of that arg. If you provide
multiple "-", the first will be used and the others simply ignored.

If you pass "-" without piped stdin, it will be treated as a literal
filename.