[3074] Add logging
[3074] Add logging
[3074] Add logging
[3074] Unsubscribe from earlier CC session first
[3191] Subnet4 now stores siaddr
[3191] Parser is now able to parse next-server parameter
[3186] Initial classes in DHCP user check hook library

Adds new source tree branch, bind10/src/hooks, for maintaining
custom hooks libraries.  Adds bare bones implementation of UserRegistry
for use in custom DHCP hooks library.
[3191] Unit-tests for siaddr configuration
[3191] User's Guide updated
[3186] Added support for loading user data from file

Added UserFile derivation of UserDataSource which allows the UserRegistry
to be populated from a JSON formatted file.
[3186] Fixed a couple of minor errors found on Fedora

Needed an include of stdint.h and to initialize the UserDataSource::open_flag_
in its constructor.
[3186] Implemented initial callouts for subnet_select

Added initial callout functions and ability to load the user_chk hooks library.
[3186] user_chk hook selects subnet4 based on user registry.

Interrim checkin. Subnet4 selection is working.
[3186] user_chk hook selects subnet6 based on user_registry

Interrim checkin. Subnet6 selection is working and user check outcome is
being written out to file, "/tmp/user_check_output.txt".
[3186] Adding logging to user_chk hook libraryU

Replaced couts with isc::log mechanisms in user_chk DHCP hook library.
[3186] Disabled use of isc::log logging in user_chk hook library.

Interrim checkin. Use of isc::logging has been disabled.  Initializing
the dictionary (MessageInitializer::loadDictionary) cores on Debian. This
has something to do with the libary being opened and closed without calling
load during Kea configuration parsing and the later reopening the library.
See trac#3198.
[3191] Changes after review:

 - new negative unit-test for next-server written
 - comments update (capitalized!)
 - indents corrected
 - no more raw pointers
 - ChangeLog added
[3191] Unnecessary comment removed.
[3186] Added doxygen commentary to user check hook lib

Added doxygen throughout, cleaned up unit tests.  Removed CLIENT_ID type
as it was unnecessary.
[3186] Added test_users_err.txt test file for user_check lib

Added unit test data file, overlooked with prior commit. Also
added missiong include to user_file.cc and took extraneous debug
cout.