taskd.pl: avoid unnecessary 30s wait for throttle proceed

taskd.pl: avoid unnecessary 30s wait for throttle proceed

Switch from 'do {} until ()' to 'until () {}' so that all checks
are made before entering the 30s sleep-until-something-happens wait.

What can happen is that the proceed signal arrives before entering
the 'do {} until ()' loop causing an unnecessary 30s delay before
acting on it.  Switching to 'until () {}' does not completely
eliminate the problem as there will always be a race between testing
for the end conditions and entering the sleep-until-something-happens
before a signal comes in, but this change greatly reduces the
opportunity for it to happen.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>