environment: Improve '--help' output. * guix/scripts/environment.scm (show-help): Align --pure and --search-paths with other long options; add 'newline' call before 'show-bug-report-information' call.
scripts: Add 'environment' command.
* guix/scripts/environment.scm: New file.
* Makefile.am (MODULES): Add it.
* doc/guix.texi ("Invoking guix environment"): New node.