services: Add 'linux-bare-metal-service-type'.

* gnu/services.scm (modprobe-wrapper): Remove.
  (activation-script): Do not use it.  Remove calls to
  'activate-modprobe' and 'activate-ptrace-attach' in gexp.
  (%modprobe-wrapper, %linux-kernel-activation,
  linux-bare-metal-service-type, %linux-bare-metal-service): New
  variables.
* gnu/system.scm (essential-services): Add %LINUX-BARE-METAL-SERVICE to
  the list, unless CONTAINER? is true.