gnu: Move helper code to (gnu system …) modules.

* gnu/packages/grub.scm (<menu-entry>, grub-configuration-file): Move
  to...
* gnu/system/grub.scm: ... here.  New file.
* gnu/packages/linux.scm (<pam-service>, <pam-entry>,
  pam-service->configuration, pam-service->directory,
  %pam-other-services, unix-pam-service): Move to...
* gnu/system/linux.scm: ... here.  New file.
* gnu/system/vm.scm (passwd-file): Move to...
* gnu/system/shadow.scm: ... here.  New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add
  gnu/system/{grub,linux,shadow}.scm.