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
Differential Revision:	https://reviews.freebsd.org/D18339

Remove unused token that was added in r348235.