execute: cleanup use of ERRNO_IS_PRIVILEGE()

execute: cleanup use of ERRNO_IS_PRIVILEGE()

Given that ERRNO_IS_PRIVILEGE() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the arguments passed to ERRNO_IS_PRIVILEGE() are the values
returned by set_oom_score_adjust() and set_coredump_filter() which are
not expected to return any positive values, but let's be consistent
anyway and move the ERRNO_IS_PRIVILEGE() invocations to the branches
where the return values are known to be negative.