Split the current section 'Network' into two sections.

The current section 'Network' is split into two new sections; the 'NetDiscovery'
section to install the NetDiscovery task and the 'NetInventory' section to
install the NetInventory task.

This commit references #2038.
New functions for management sections.

New file ./NSIS/FusionInventory-Agent/Include/SectionFunc.nsh with the
following definitions, macros and functions.

 * SelectSection
 * UnselectSection
   (variations on the original)

 * GetSectionsNumber
 * SetSectionsNumber
   (for the sake of clarity)

 * GetCurrentSelectedFlagsOfSections
 * GetSavedSelectedFlagsOfSections
 * SaveCurrentSelectedFlagsOfSections
   (only save the state of normal sections, not groups)

This commit closes #2023.
Split the current section 'Network' into two sections.

The NetDiscovery and NetInventory sections are now inder-dependent.

This commit closes #2038.