GitLab

 - Also code updated to match changes from 2980

Conflicts:
	src/lib/hooks/callout_handle.h

This reduces the risk of someone taking a copy and being left with
a "dangling pointer" to a callout manager.

On Ubuntu, DLLs have the suffix .so: on OSX, the suffix is .dylib

* Catch user-library-generated exceptions.
* Remove hook registration function (no longer needed with a singleton
  ServerHooks object.
* Miscellaneous documentation changes.

 - pkt6_receive, subnet6_select, lease6_select, pkt6_send
 - framework updated
 - some unittests implemented

This manages LibraryManagers created for the libraries loaded.

LibraryManager can now load and unload libraries.  Multiple
libraries can be loaded at the same time and the callouts
execute correctly.

runUnload() still has to be added.

Added ability to run the load function and checked that it can
register callouts.

Also started to add logging.