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Also, added documentation to the Host class.

The modifications to the staging configuration are performed on the pointer
returned by getCfgOptionDef.

This change affects many files using the Configuration object and types
derived from it.

Comments have been improved in one file.  The other contains a test
that alters logging, so a test fixture has been added that ressets
the logging back to the default in its destructor.

 - KEA_LOCKFILE_DIR_FROM_BUILD renamed to KEA_LOCKFILE_DIR.
 - KEA_FROM_BUILD_LOCALSTATEDIR, KEA_LOCKFILE_DIR_FROM_BUILD removed
 - It is possible to disable lockfile now

Some unit tests in dhcpsrv altered the logging configuration but
never reset it. That configuration was then used for subsequent
tests and made them overly verbose.

Previously, the logger configuration was applied (to log4cplus) as soon as
it was parsed. We decided in the jabber discussion that the logger
configuration should only be applied if the whole configuration goes
through. So, the logger configuration is now 2-stage: parsing and commit
as for other configuration parameters.

This is trivial fix so as I am committing it on my own discretion. Also,
this fix had already been reviewed and applied on master branch but
apparently disappeared after the recent git migration.