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This change includes the code implemented by the pull request
submitter as well as Marcin's fixes/changes on top of it, but
without a review.

Also removed uint32_t cast operator from IOAddress to remove
ambiguity when calling this new function.

 - Dhcp4Srv::vendorClassSpecificProcessing removed
 - User's Guide updated
 - disabled obsolete test (will need to be rewritten to take advantage
   of new classification code)
 - classes definitions now use strings for server-name and filename
 - unused configuration removed from unit-tests
 - textFixedFields is now documented
 - Unit-tests now have short descriptions

For the DHCPINFORM case set destination port 68 if:
- direct message sent without ciaddr,
- ciaddr present

Also updated unit test to verify port number for the directly
sent DHCPINFORM without ciaddr.

This is a small performance improvement.

A call to HooksManager::getHooksManager().unloadLibraries() was added to
the destructors Dhcpv4Srv::~Dhcpv4Srv() and Dhcpv6Srv::~Dhcpv6Srv(). This
ensures hook libraries are unloaded prior to static objects.

- No additional check for HR mode in findReservations
- Additional commentary in the loop collecting host ids.