Do not disable unused parameters warnings in tests.

Do not disable useful warnings in generic header files, they might
actually point us to bugs in the tests. We do want to minimise warnings
during compilation, but this should be done by fixing (or suppressing
with ARG_UNUSED) each case individually.
Prioritize uptime alternatives in headers, not in implementation.

Also simplified the logic a bit, since libpcre is now a hard dependency,
so we can always parse the "uptime" command output thus uptime
calculation will never fail.