Having access to this function is useful in a variety of cases:
- hand-crafting PDUs for unit tests
- implementing extensions of the protocol
- reusing the code for other purposes (e.g. firmware integrity check)
I think this makes the code easier to read, since you can tell whether each
function call will actually `send` some bytes over the wire, or if it will
just `put` some bytes into the buffer.
`nmbs_client_create` accepts a `const nmbs_platform_conf* platform_conf` with configuration. Without reading the source code, it is not clear whether or not the object pointed to by `platform_conf` can be declared on the stack, or if it needs to be kept alive for the duration of the program (as would be the case if the implementation simply kept a pointer to the configuration instead of copying it by value). This PR adds an assurance that it is OK to discard the platform configuration after calling `nmbs_client_create`. Same idea for `nmbs_server_create`.
