BUG/MINOR: resolvers: reject null "timeout resolve" and "timeout retry" values

BUG/MINOR: resolvers: reject null "timeout resolve" and "timeout retry" values

Both timeouts are parsed with parse_time_err() which accepts zero. But
"timeout resolve" is the delay between two resolutions of a same name,
so with a null value, a resolution is restarted as soon as it completes,
from the same task wakeup, and the resolvers task spins at 100% CPU
flooding the nameservers. With a null "timeout retry", all retries of a
resolution are sent at once without waiting for any response, which
defeats their purpose.

This can be reproduced with "timeout resolve 0ms" (which doesn't even
emit the "suspiciously small" warning thanks to the unit) and a server
resolving a name answered by the nameserver: 68225 queries were sent in
2 seconds, versus 2 with "timeout resolve 1s".

Let's reject null values for these two timeouts.

This patch should be backported to all stable branches.