Track CONFIG_MODULE_SRCVERSION_ALL and behave accordingly

[evelikov]

The CONFIG_MODULE_SRCVERSION_ALL option controls if the kernel creates srcversion information for modules or not. If the toggle is off, or the module is built-in the field is missing.

Currently we have a few interesting assumptions in both the tests (which misleadingly claims "On debian, no srcversion ...") and dkms codebase. To fix properly both places should first check if the config option is set and then acting accordingly.

It might be worth splitting out the modinfo version+checksum and srcversion handling in dkms itself. At the moment it's pure magic:

• get_module_verinfo gives you two lines - first has the version (which can be missing, if the driver doesn't define a version), and srcversion (or if missing a checksum). checksum is seemingly part of the version field... alas there's no actual references to that
• we wrap ^^ into a function that returns a string operator + error code depending on the comparison
• some places check the code, others the string, only to call again get_module_verinfo 🤦