Dummynet does not require burst size specification

Dummynet traffic shaper does not require burst size specification and
assumes 0 if not specified. Allow user to leave burst field blank, if
not blank the must be numeric
touch up text, s/nat/NAT/
Merge pull request #791 from jean-m-cyr/RELENG_2_1

Dummynet does not require burst size specification
Merge pull request #792 from razzfazz/RELENG_2_1

add option to send prefix hint for requesting desired prefix length for ...