check_dig: patch to make dig honor -t option

When a timeout value is specified with the -t option, dig will sometimes
timeout before the timer is actually reached.

The problem occurs because the check_dig plugin does not pass the specified
timeout value to dig, leaving dig to timeout with it's default value which
seems to be around 10-15seconds.

To reproduce:
time ./check_dig -H 127.0.0.2 -l www.google.com -t 30

It will not run for 30secs, which is the expected behaviour.

The following will work, because the timeout is less than the default dig
timeout, so the plugin cancels the dig command:
time ./check_dig -H 127.0.0.2 -l www.google.com -t 2

This fix passes the timeout value to dig, and sets the number of retries which tends to vary from system to system by default.

Closes #1168
check_dig: timeout_interval is a global variable
which is already set to 10 seconds
check_dig: stick with integer devision

This change saves us from having to link check_dig against "libm"