expr: limit: avoid huge rodata array

commit 10e0890e ('src: operational limit match') creates huge
array, increasing libnftables binary size. Use switch statement
instead.

Based on patch from Florian Westphal, for nft.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
expr: limit: s/seconds/second/

my fault, spotted by Phil Oester.

Signed-off-by: Florian Westphal <fw@strlen.de>