RMNET support: allow disabling, and avoid relying on kernel headers

There may be systems using a new enough kernel that would have rmnet support as seen in the installed kernel headers, but in which CONFIG_RMNET was not really enabled, and so there is no true rmnet support during runtime. We now allow disabling rmnet support completely at configure time, so that all that logic is completely skipped if we know the kernel won't support it.

We also avoid relying on installed kernel headers, as these are not always available when building the library. Instead, we provide a custom header defining the rmnet related bits, which will be used only when the system-provided header isn't found.