GitLab

Addressed review comments which were largely minor. Limited use of
extern C linkage to only the callout functions themselves. Added
a dox page describing the library.  Added namespace user_chk.

Vendor option default values are now taken from default users if the
client is not registered.  Registry input file user ids can now contain
colons as delimiters allowing cut-and-paste from outcome file.

Mostly minor cosmetics. Added logic to select callouts to
simply return if no subnets are configured.

Added doxygen throughout, cleaned up unit tests.  Removed CLIENT_ID type
as it was unnecessary.

Added UserFile derivation of UserDataSource which allows the UserRegistry
to be populated from a JSON formatted file.

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.