MINOR: stick-tables: add "ipv4" as an alias for the "ip" type

MINOR: stick-tables: add "ipv4" as an alias for the "ip" type

However the doc purposely says the opposite, to encourage migrating away
from "ip". The goal is that in the future we change "ip" to mean "ipv6",
which seems to be what most users naturally expect. But we cannot break
configurations in the LTS version so for now "ipv4" is the alias.

The reason for not changing it in the table is that the type name is
used at a few places (look for "].kw"):
  - dumps
  - promex

We'd rather not change that output for 3.2, but only do it in 3.3.
This way, 3.2 can be made future-proof by using "ipv4" in the config
without any other side effect.

Please see github issue #2962 for updates on this transition.