ip_reass: provide sysctl MIB returning IP fragment TTL

For now it is read-only, but eventually the cycle that goes over
all fragments should be refactored and this MIB should also become
read/write.

This MIB will allow SNMP daemons to implement MIB-II ipReasmTimeout MIB
straightfoward. Right now net-snmp compilation is broken by 1922eb3e9c2.
The base system bsnmpd is not broken just because it ignored PR_SLOWTIMO,
and thus always returned incorrectly doubled value for ipReasmTimeout.