[trac2945] miscellaneous doxygen warnings worked on

Still have lots to do.
Some of these marked with TODO indicating needing further work
(but better than it was before).
[2945] Add missing doxygen comments
[2945] Update Doxygen reference to isc::ConfigData::getFullConfig()
[2945] Update Doxygen reference to isc::ConfigData::getFullConfig() (contd.)
[3203] Client classes in packets added.
[3203] DHCPv4 Vendor-class option is now a string.
[3203] Client classification added in DHCPv4.
[3203] v6 vendor-class option updated, unit-test added
[3203] Comment in Pkt4 about client classes updated.
[3203] v6 vendor class is not intepreted as client class.
[3203] Minor tweaks in v4 client classification
[3203] Developer's guide update
[3203] Doxygen errors fixes
[3288] Implemented OS-specific function which opens socket on multicast.
[3288] Removed accidentally committed file.
[3292] Fixed option definition class to handle booleans carried as strings.
[3292] Fixed configuration of options holding boolean values in DHCPv4.
[3292] Fixed configuration of options holding boolean values for DHCPv6.
[3203] Changes after review:

 - Dhcpv{4,6}Srv::classifyPacket() is now better commented.
 - constants are now used instead of magic numbers
 - Pkt{4,6}::addClass is now better commented
[3292] Removed redundant checks in DHCP srv unit tests.