Source code duplication and refactoring for engine related code.

The source code for the engine_ module is actually located in multiple source codes, without clear reason. Both enforcer and signer contain the cfg.[ch] and engine.[ch] source files that each contain part of the engine_ module source code.

In source code signer/src/daemon/cfg.[ch] and enforcer/src/daemon/cfg.[ch] the symbols engine_config(), engine_config_check(), engine_config_print(), engine_config_cleanup() are duplicate and are partially the same, and partially differ to handle the differences between signer and enforcer and partially differ because of diverged code base that could/should be the same.

There is also a thought that there can be multiple engine's working. This can hardly be true for a single enforcer or signer. When running an enforcer or signer in a single instance one could argue for this, but one could better argue there can be only a single abstract engine and signer/enforcer are independent of the outer 'shell' that controls daemonization, configuration and scheduling set-up.