guix package: Add optional argument to --search-paths.

guix package: Add optional argument to --search-paths.

* guix/scripts/package.scm (search-path-environment-variables): Add #:kind
  parameter.  Pass it to 'environment-variable-definition'.
  (display-search-paths): Add #:kind parameter and pass it to
  'search-path-environment-variables'.
  (%options): Add an optional parameter for "--search-paths".
  (guix-package)[process-query]: Handle it.
* tests/guix-package-net.sh: Adjust existing test.
* tests/guix-package.sh: Adjust existing tests and add new test.
* doc/guix.texi (Invoking guix package): Document it.