shared: cleanup separation and transition between errno and nmerr numbers
The native error numbers (from <errno.h>) and our nmerr extention on top of them are almost the same. But there are peculiarities. Both errno and nmerr must be positive values. That is because some API (systemd) like to return negative error codes. So, a positive errno and its negative counter part indicate the same error. We need normalization functions that make an error number positive (these are nm_errno() and nm_errno_native()). This means, G_MININT needs special treatment, because it cannot be represented as a positive integer. Also, zero needs special treatment, because we want to encode an error, and zero already encodes no-error. Take care of these special cases. On top of that, nmerr reserves a range within native error numbers for NetworkManager specific failure codes. So we need to transition from native numbers to nmerr numbers via nm_errno_from_native(). Take better care of some special cases and clean them up. Also add NM_ERRNO_NATIVE() macro. While nm_errno_native() coerces a value in the suitable range, NM_ERRNO_NATIVE() asserts that the number is already positive (and returns it as-is). It's use is only for asserting and implicitly documenting the requirements we have on the number passed to it.
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