xshared: Prefer xtables_chain_protos lookup over getprotoent

xshared: Prefer xtables_chain_protos lookup over getprotoent

When dumping a large ruleset, common protocol matches such as for TCP
port number significantly slow down rule printing due to repeated calls
for getprotobynumber(). The latter does not involve any caching, so
/etc/protocols is consulted over and over again.

As a simple countermeasure, make functions converting between proto
number and name prefer the built-in list of "well-known" protocols. This
is not a perfect solution, repeated rules for protocol names libxtables
does not cache (e.g. igmp or dccp) will still be slow. Implementing
getprotoent() result caching could solve this.

As a side-effect, explicit check for pseudo-protocol "all" may be
dropped as it is contained in the built-in list and therefore immutable.

Also update xtables_chain_protos entries a bit to align with typical
/etc/protocols contents. The testsuite assumes those names, so the
preferred ones prior to this patch are indeed uncommon nowadays.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Florian Westphal <fw@strlen.de>