ipfw: make algo name argument optional for some table types

Most of table types currently supported by ipfw have only one
algorithm implementation. When user creates such tables, allow
to omit algo name in arguments. E.g. now it is possible:
ipfw table T1 create type number
ipfw table T2 create type iface
ipfw table T3 create type flow

PR: 233072
MFC after: 1 week
Sponsored by: Yandex LLC