Add burst option to trafficshaper/pipe

[jschellevis]

For certain trafficshaping scenario's it is usefull to have a burts option to be able to use full bandwidth for a limited time size.

--- burst section pipe of man page ipfw ---

burst size
If the data to be sent exceeds the pipe's bandwidth limit (and
the pipe was previously idle), up to size bytes of data are
allowed to bypass the dummynet scheduler, and will be sent as
fast as the physical link allows. Any additional data will be
transmitted at the rate specified by the pipe bandwidth. The
burst size depends on how long the pipe has been idle; the effec-
tive burst size is calculated as follows: MAX( size , bw *
pipe_idle_time).