* gnu/bootloader.scm: New file.
* gnu/bootloader/extlinux.scm: New file.
* gnu/bootloader/grub.scm: New file.
* gnu/local.mk: Build new files.
* gnu/system.scm: Adapt to new bootloader api.
* gnu/scripts/system.scm: Adapt to new bootloader api.
* gnu.scm: Remove (gnu system grub) and replace by (gnu bootloader) and (gnu
bootloader grub) modules.
* gnu/system/grub.scm: Moved content to gnu/bootloader/grub.scm.
* gnu/system/vm: Replace (gnu system grub) module by (gnu bootloader).
* gnu/tests.scm: Ditto.
* gnu/tests/nfs.scm: Ditto.
* gnu/services/shepherd.scm (<shepherd-service>)[imported-modules]:
Remove.
(%default-imported-modules): Make private.
(shepherd-service-file): Use 'with-imported-modules'.
(shepherd-configuration-file): Remove 'modules' and the calls to
'imported-modules' and 'compiled-modules'. Use
'with-imported-modules' instead.
* doc/guix.texi (Shepherd Services): Adjust accordingly.
* gnu/services/base.scm (file-system-shepherd-service): Use
'with-imported-modules'. Remove 'imported-modules' field.
* gnu/system/mapped-devices.scm (device-mapping-service-type): Remove
'imported-modules'.
(open-luks-device): Use 'with-imported-modules'.
* gnu/tests.scm (marionette-shepherd-service): Remove 'imported-modules'
field and use 'with-imported-modules'.
* gnu/tests.scm (<marionette-configuration>): New record type.
(marionette-shepherd-service): Argument now is a <marionette-configuration>.
(marionette-operating-system): Adjust accordingly. Add #:requirements
parameter and honor it.
* gnu/tests.scm (<system-test>): New record type.
(write-system-test, test-modules, fold-system-tests)
(all-system-tests): New procedures.
* gnu/tests/base.scm (%test-basic-os): Turn into a <system-test>.
* gnu/tests/install.scm (%test-installed-os): Likewise.
* build-aux/run-system-tests.scm (%system-tests): Remove.
(run-system-tests): Use 'all-system-tests'.
