Convert the tcp segment reassembly queue to UMA and limit the maximum
amount of segments it will hold.

The following tuneables and sysctls control the behaviour of the tcp
segment reassembly queue:

net.inet.tcp.reass.maxsegments (loader tuneable)

specifies the maximum number of segments all tcp reassemly queues can
hold (defaults to 1/16 of nmbclusters).

net.inet.tcp.reass.maxqlen

specifies the maximum number of segments any individual tcp session queue
can hold (defaults to 48).

net.inet.tcp.reass.cursegments (readonly)

counts the number of segments currently in all reassembly queues.

net.inet.tcp.reass.overflows (readonly)

counts how often either the global or local queue limit has been reached.

Tested by: bms, silby
Reviewed by: bms, silby