main: Bugfix: Print all command line help text to stderr, not stdout

[fxkr]

Previously some was printed to stderr, some was printed to stdout.

This caused the help text to be garbled sometimes, e.g. in certain terminals. Before fix:

After fix:

(Actually I first noticed this when running vegeta on a remote system using something like fabric, where stdout and stderr are captured separately and returned as two strings, so any interleaving gets lost.)

Background

Checklist

• Git commit messages conform to community standards.
• Each Git commit represents meaningful milestones or atomic units of work.
• Changed or added code is covered by appropriate tests.

[tsenart]

Thanks!