mbuf: Fix jcluster support

- Free the jcluster to the correct objcache, which solves "jcluster mbuf"
  objcache exhaustion.
- By default, set the amount of the jclusters to half the amount of the
  normal clusters.  jcluster will be used on TCP sending path in the later
  commit to improve TSO performance for 10Ge.
- Add mbuf stat for jcluster; adjust netstat(1) to show it.
- Add m_getlj(), which will be used on TCP sending path in the later
  commit to improve TSO performnace for 10Ge.
tcp: Use mbuf jcluster (4K) in sosendtcp(); improve 10Ge TSO performance

For the Myricom PCIE-8AL-C I have tested, this gives ~900Mbps performance
boost using 1500 MTU when TSO is enabled (from 7.7Gbps to 8.6Gbps).

Using mbuf jcluster could:
- Reduce the number of TX descriptors needed for one TSO packet.
- Let the NIC chip perform longer large data burst.

I believe this is the main reasons for the 10Ge performance boost.

Reduce sosend_agglim from 3 to 2, which means 8K aggregation (was 6K
aggregation) before user thread dispatches the sending buf to netisr.

net.inet.tcp.sosend_jcluster is added to enable this feature; it is
enabled by default.