Convenience API for configuration handling

[penguineer]

Extend the API for better configuration handling, such as:

• Print the current configuration
• Just load a configuration from file and return the created configuration object
• Move a pre-loaded configuration object to the agent

[penguineer]

Printing has already been started there:

mqtt-tools/mqtt-clock.c

Line 202 in 055f435

printf("Agent configuration:\n"

Should be moved to a library function.

[penguineer]

Moving pointers into the object leads to a problem when pointers have not been created with malloc: The library will try to free the pointer, but fails (with undefined behaviour) on stack pointers.

However, it is a common pattern to provide this structure via stack pointer (from a local or static variable). I wonder if a warning in the API documentation is sufficient to keep developers from just providing a pointer to a stack variable or if there should be a mechanism to distinguish these, i.e. have two functions and a flag to denote whether the pointer must be free'd.

[LeSpocky]

Why should the library free things it did not allocate by itself anyway? That should always be responsibility of the caller. So: if the library allocates some config object by itself (maybe a copy of an existing object), the library can free it again unless otherwise documented. I would say freeing things allocated outside of a library is bad practice?

[penguineer]

The idea is that clean-up is done completely by the library, so handling will be easier for the user.
(Clean-up could take place in some abort routine or at a place where information about the configuration's origin (loaded from file, from command line) is no longer known and would otherwise have to be tracked.)

So,

• either the users do resource management by themselves, i.e. allocate and free ressources for the agent and the configuration object (which seems kind of redundant)
• or the user hands pointer ownership to the library (similiar to unique-ptr in C++) and clean-up is done automatically for the configuration object.

I'll propose a solution for both use cases.

[penguineer]

Resource management has been implemented in #12, which concludes the above discussion for now.