Configuration cleanup: use constructor/destructor semantics.

* src/server/conf-file.h (struct datatype): Replaced the freer
attribute with ctor and dtor attributes.  All users updated.
Added some documentation.
(free_config): New function.
* src/server/conf-file.c (init_init): Call the constructors for
all parameters.
(free_config): New function, that calls the destructors for all
parameters.
(ctor_string): New static function. 
(dtor_string): New static function, which replaces
unassign_string.
(assign_string): Expect the object to already be constructed.
(unassign_string): Removed.
(cf_string): Use ctor_string and dtor_string.
* src/server/server-config.c (cf_log_param): Updated for new
struct datatype.
(cf_jubel): Ditto.
(cf_ident_param): Ditto.
(free_configuration): Use free_config to do the work.