This allows for direct program invokation without needing a shell to act
as a command interpreter.

* guix/scripts/environment.scm (%default-shell): New variable.
  (show-help): Adjust description.  Remove '--exec' reference.
  (%default-options): Use '%default-shell'.
  (%options): Adjust '--exec' to run command via the default shell.
  (parse-args): New procedure.
  (guix-environment): Use 'parse-args'.  Use 'system*' instead of
  'system'.
* tests/guix-environment.sh: Add test for '--' command invokation.
* doc/guix.texi ("Invoking guix environment"): Use new syntax.  Remove
  '--exec' documentation.