gluon-core: preserve explicitly marked network and system sections

A section can be marked as preseved by setting the gluon_preserve option
to 1. In addition the following conditions must hold:

- The preserved section must not already exist after OpenWrt's and
  Gluons setup scripts run. Modifying existing sections is currently
  unsupported.
- Preserved sections must be named, so it can be detected whether a
  section conflicts with a preexisting one.