[3591] logger_unittest_support no longer overwrites KEA_LOCKFILE_DIR_FROM_BUILD
[3591] Logging initialization cleaned up
[3591] Logging variables simplified

 - 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
[3591] Print error to stderr after failing lock file creation.
[3591] Default logging refactored.
[3591] Unit-tests for new variables implemented.
[3591] Documentation updated.
[3591] Unnecessary variable set removed.
[3588] Implemented CfgOptionDef class to represent user-defined option defs
[3536] Logging section in User's Guide updated.
[3536] logging example in kea.conf in keactrl fixed
[3588] Return pointer to the RW option definition configs, not a reference.
[3588] Removed the setCfgOptionDef modifier.

The modifications to the staging configuration are performed on the pointer
returned by getCfgOptionDef.
[3591] Fix for DESTDIR installation hook
[3536] Missing coma added.
[3591] Small edits of logging documentation during review.
[3536] Changes after review:

 - Logging section updated.
 - Missing coma in kea.conf.pre added
 - ${localstatedir}/log is now created
Merge branch 'trac3536' of ssh://git.kea.isc.org/git/kea into trac3536
[3536] Disabled ddns in keactrl.conf.in
[3591] Shell tests renamed.