[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).
Don't crash with missing TSIG name
Remove hard-coded (and unecessary) TSIG key from error message.
[2945] Add missing doxygen comments
[2945] Update Doxygen reference to isc::ConfigData::getFullConfig()
[2945] Update Doxygen reference to isc::ConfigData::getFullConfig() (contd.)
[3239] Update comment
[3239] Expand documentation on configuration data type syntax
[3239] Update exception messages returned when types mismatch

We include the type of the received value so that the user can see the
type mismatch more clearly from the returned exception message.
[2566] Include logging maintenance guide into BIND 10 Developer's Manual

Convert the old logging "README" file to Doxygen format.  The opportunity
was taken to expand and edit it at the same time.
[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
[2566] Review - minor potential cleanups