[binhvq]

Fixes #345.

When stdout is /dev/null, parse_params set quiet, which is also what -s sets and what hides error messages. So cmp a missing > /dev/null exited 2 with nothing on stderr, and cmp -l -s a b > /dev/null was accepted.

The redirect now has its own flag. It only skips the difference and EOF reports; errors are hidden by -s alone, and the -l/-s check runs first.

The test from the issue doesn't compile (assert_cmd::Command has no stdout()), so the new tests use std::process::Command like cmp_fast_paths.