crontab: multiple :time, :tasks values are supported

The primary purpose is to support multiple time entries for some
task, and/or to support multiple tasks at some point (rare!). E.g,

  :time => [@reboot, @daily], :task => [test1, test2]

will expand to 2x2 = 4 entries in /etc/crontab:

  @reboot test1 ; @reboot test2
  @daily  test1 ; @daily  test2