Remove redundant GNU/POSIX feature macros
README.md: fix spelling
Add support for overwriting argv0 with an env var

This is useful for wrapper scripts that want to hide the full path
to the program while only relying on POSIX scripting functionality.
(`exec -a` is a bashism)
Improve options listing in the man page

- Separate alternative options with a comma.
- Only highlight the options, not their parameters.