Add `missing` and `or-flush` options to "ipfw table <NAME> create"


Add missing and or-flush options to "ipfw table <NAME> create"
command to simplify firewall reloading.

The missing option suppresses EEXIST error code, but does check that
existing table has the same parameters as new one. The or-flush option
implies missing option and additionally does flush for table if it
is already exist.

Submitted by: lev
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D18339