system: Add 'multiboot-modules' field to <boot-parameters>.

system: Add 'multiboot-modules' field to <boot-parameters>.

* gnu/system.scm (<boot-parameters>)[multiboot-modules]: New field.
(read-boot-parameters): Initialize it.
(operating-system-multiboot-modules, hurd-multiboot-modules): New procedure.
(operating-system-boot-parameters): Cater for multiboot the Hurd and
initialize it; avoid initrd in that case.
(operating-system-kernel-file): Cater for for Gnumach (the Hurd) besides Linux.
(boot-parameters->menu-entry): Use it to support a multiboot <menu-entry>.