Separate zenroom_init from zencode_init on context creation

[jaromil]

Following analysis on #892 that mandates init also on argument validation, it makes sense to separate the two kinds of init: the zencode specific one (zen_init_lua in lua_modules.c) can be called afterwards when all input data is validated, this may permit faster start in some circumstances.

[jaromil]

Also quoting @matteo-cristino :

There are still some _err() calls that does not follow the logfmt specified in the conf, but these can not be directly substituted with zerror() since are zenroom or lua initialization error. Another solution should be found for these cases to report errors with the right format.