`command-name` internally uses some implementation-defined (but
common-sense) rules to simplify a filename into a displayable command
name (e.g. "C:\foo\bar.exe" into "bar" or "../foo-123.scm" into "foo-123".

Should the simplifier be exported as its own procedure, so that users of
the SRFI can call (simplify-command-name any-arbitrary-string) to
simplify command names that come from elsewhere? I didn't include this
procedure since it wasn't clear whether there is demand.