Time is up for SRFI 193 (Command lines).

As discussed the mailing list, `os-executable-file` is too complex to be
reliable. Hence I will remove it from this particular SRFI.

`os-command-line` is basically reliable, but it can be hard to get at
this information from a high-level VM like the JVM. Hence I'll remove it
as well.

As for the rest of the SRFI, we worked hard with Shiro to get the
semantics right before submitting the SRFI. They still seem OK to me,
but I'll update the rationale.

That should be all for a last-call version. Let me know if there are
other concerns remaining.

As a low-level continuation of SRFI 193, I prepared a pre-SRFI about
"Process state as bytevectors". It's here:
<https://misc.lassi.io/2020/srfi-submit/process-state-as-bytevectors.html>.
It can get:

- OS process current directory
- OS process environment variables
- OS process command line
- OS process executable filename

Feedback on that pre-SRFI is very welcome as well. We can use the SRFI
193 mailing list to discuss it until it has been submitted as a real
draft SRFI (which I'll try to do ASAP, or whenever Arthur thinks there
is enough breathing room in the SRFI queue).