system: Add 'kernel-arguments' field.

* gnu/system.scm (<operating-system>)[kernel-arguments]: New field.
  (operating-system-grub.cfg): Honor it.
  (operating-system-parameters-file): Add 'kernel-arguments' to the parameters
  file.
* guix/scripts/system.scm (previous-grub-entries)[system->grub-entry]: Read
  the 'kernel-arguments' field of the parameters file, when available.
* gnu/system/vm.scm (system-qemu-image/shared-store-script):
  Use (operating-system-kernel-arguments os) in '-append'.
* doc/guix.texi (operating-system Reference): Document it.