sd-resolve: tweak error assignment code a bit, use abs() and regular functions

sd-resolve: tweak error assignment code a bit, use abs() and regular functions

Let's tweak the assignment of errors a bit, and automatically abs()
errnos, similar to how log_error_errno() and friends does it.

Macros are fine to use, but regular functions usually preferable if
there's no reason to use macros, because they avoid multiple evaluation
and suchlike. Hence, let's just use a regular funciton for assigning
errors, instead of macros.

Follow-up for #8993