src: disentangle ICMP code types

src: disentangle ICMP code types

Currently, ICMP{v4,v6,inet} code datatypes only describe those that are
supported by the reject statement, but they can also be used for icmp
code matching. Moreover, ICMP code types go hand-to-hand with ICMP
types, that is, ICMP code symbols depend on the ICMP type.

Thus, the output of:

  nft describe icmp_code

look confusing because that only displays the values that are supported
by the reject statement.

Disentangle this by adding internal datatypes for the reject statement
to handle the ICMP code symbol conversion to value as well as ruleset
listing.

The existing icmp_code, icmpv6_code and icmpx_code remain in place. For
backward compatibility, a parser function is defined in case an existing
ruleset relies on these symbols.

As for the manpage, move existing ICMP code tables from the DATA TYPES
section to the REJECT STATEMENT section, where this really belongs to.
But the icmp_code and icmpv6_code table stubs remain in the DATA TYPES
section because that describe that this is an 8-bit integer field.

After this patch:

 # nft describe icmp_code
 datatype icmp_code (icmp code) (basetype integer), 8 bits
 # nft describe icmpv6_code
 datatype icmpv6_code (icmpv6 code) (basetype integer), 8 bits
 # nft describe icmpx_code
 datatype icmpx_code (icmpx code) (basetype integer), 8 bits

do not display the symbol table of the reject statement anymore.

icmpx_code_type is not used anymore, but keep it in place for backward
compatibility reasons.

And update tests/shell accordingly.

Fixes: 5fdd0b6a0600 ("nft: complete reject support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>