fix clocktool lagging behind and skipping seconds

from time to time (...) time() might be off to gettimeofday() by a
second. the reason for this is that time() is usually implemented
by just returning the field 'second' of the struct that represents
the clock inside the kernel. gettimeofday() on the other hand also
takes the 'fraction' field (mostly 'nanoseconds') into account and
thus is closer to the current time than time().

the result of using time() was a perceived 'lag', sometimes the
clocktool even skipped a second. by using FbTk::FbTime()::system()
instead fixes the issue.
detect minute-based strftime-formats (again)

the lag / skipping of the clock was not caused by faulty timer code
on fluxbox's side but by the behavior and inner workings of time().
since this is fixed now (913244789f) we can now rollback ec7fe513c8
and detect strftime-formats which need intervals of seconds or minutes.

minor: the small change to FbTk::Timer::setTimeout() reduces one
start() / stop() cycle for a running timer.
cosmetic, less code
allow a timeout of a full 'unit'

the -1 in the FbTk::FbTimer::remainingNext() function was pointless in the
first place anyway: reducing the timeout by just 1 microsecond improves
nothing (in this case). if the timer triggers exactly at a full unit (second)
then it's correct to wait for the full next unit.