ebtables: Improve invalid chain name detection

ebtables: Improve invalid chain name detection

Fix several issues:

- Most importantly, --new-chain command accepted any name. Introduce
  ebt_assert_valid_chain_name() for use with both --new-chain and
  --rename-chain.
- Restrict maximum name length to what legacy ebtables allows - this is
  a bit more than iptables-nft, subject to be unified.
- Like iptables, legacy ebtables rejects names prefixed by '-' or '!'.
- Use xs_has_arg() for consistency, keep the check for extra args for
  now.

Fixes: da871de2a6efb ("nft: bootstrap ebtables-compat")
Signed-off-by: Phil Sutter <phil@nwl.cc>