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.
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