tcp: Aggregate the mbuf in sosendtcp() a little bit

tcp: Aggregate the mbuf in sosendtcp() a little bit

This greatly reducse the ipi interrupts caused by ipi sending (both
domsg and sendmsg), thus improves overall performance a bit.

net.inet.tcp.sosnd_agglim is added to tune how much mbuf should be
aggregated; it is default to 2.  Setting this sysctl to 1 restores
the old behaviour: one full mbuf at a time.

On Phenom 9550 (4 core, 2.2GHz):
8 parallel netperf -H 127.0.0.1 -P0 (4 runs, unit; Mbps)

                                              IPIs/s (sum of 4 core)
 1 mbuf  6735.98  6903.13  6971.89  7056.66   ~400K
 2 mbuf  7675.47  7757.28  7815.45  7514.50   ~240K
 4 mbuf  7895.33  7584.22  7704.12  7723.33   ~180K
 8 mbuf  8006.94  8077.87  7701.23  8061.12   ~120K
16 mbuf  8151.68  8023.03  7972.42  8046.13   ~100K

The default value (2) for the sosnd_agglim improve the whole
performance by ~10%.  IPI rate is also reduce greatly.

It has no obvious impact on 1000BaseT or 100baseTX network performace.