shell: Add '--export-manifest'.

* guix/scripts/shell.scm (show-help, %options): Add '--export-manifest'.
(manifest-entry-version-prefix, manifest->code*)
(export-manifest): New procedures.
(guix-shell): Honor '--export-manifest'.
* tests/guix-shell-export-manifest.sh: New file.
* Makefile.am (SH_TESTS): Add it.
* doc/guix.texi (Invoking guix shell): Document '--export-manifest'.
(Invoking guix environment): Link to it.
(Invoking guix pack): Likewise.

Makefile.am

@@ -572,6 +572,7 @@ SH_TESTS =					\
   tests/guix-environment.sh			\
   tests/guix-environment-container.sh		\
   tests/guix-shell.sh				\
+  tests/guix-shell-export-manifest.sh		\
   tests/guix-graph.sh				\
   tests/guix-describe.sh			\
   tests/guix-repl.sh     			\