env: adjust diagnostics provided for shebang usage

env: adjust diagnostics provided for shebang usage

* src/env.c (main): Don't process '-' specially since
that causes an issue on the openbsd getopt implementation
where a lone '-' is now processed as an option, and anyway
it doesn't particuarly help diagnosing common shebang
usage issues.  Also don't restrict the extra diagnostics
for shebang usage to the case with 3 arguments, as
further arguments can be passed to a script.
* tests/misc/env-S.pl: Adjust accordingly.