ebtables: Merge libebt_limit.c into libxt_limit.c

ebtables: Merge libebt_limit.c into libxt_limit.c

Both extensions were very similar already, but now that they both are
translated into native nftables code, their actual difference (i.e.
match size) doesn't matter anymore.

This change comes with one caveat: Since ebtables limit match is not in
its own file anymore, match preloading automatically also loads the
NFPROTO_UNSPEC limit match. This is not a problem per se since match
lookup will prefer the family-specific one, but when parsing unknown
options, a match without 'parse' callback is encountered. Therefore
do_commandeb() has to check existence of that callback prior to
dereferencing it.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>