Bring back support for checking tables via "ipfw -n".

Currently we have different table key types which can easily interfere
with each other (numbers and IPv4 address, interface names and hostnames,

flows and hostnames/addresses).

This conflicts are solved by [auto-]creating _typed_ tables, so after
table is created, only keys of given type can be inserted to that table.
ipfw(8) consults with kernel about key/value type for particular table so
it knows key/value interpretation.
However, we have 2 cases (adding entries to non-existing table and

parsing configuration file via `ipfw -n`) when kernel is unable to
provide us table info we need. Fix the latter case by partially importing
old `table_fill_xentry()` parse function responsible for guessing key type.

Sponsored by: Yandex LLC