add --language argument

add --language argument

* module/ice-9/command-line.scm (*usage*): Make usage of capitalization
  and sentences consistent (lower-case and semicolons, as in ls
  --help).
  Be less specific about languages (Scheme is the default but not the
  only language).
  Document --language.
  (load/lang, eval-string/lang): New helpers.
  (compile-shell-switches): Parse a --language argument, and use it to
  set (current-language).