tcp sendspace conservation

[Brad Harvell <dfbrad@xxxxxxxxxxx>]

Allow use of larger tcp send buffers with less concern for mbuf
consumption.  Adjust send buffers in relation to the active send
window of the tcp connection; use only the amount of buffer that
is justified.  I've had very pleasing results with this code on
heavily loaded production FreeBSD 4.x machines.

adds the following sysctls:

net.inet.tcp.sendspace_adjust_enable: Adjust send buffer in relation to active send window
net.inet.tcp.sendspace_adjust_min: Minimum adjusted send buffer size
net.inet.tcp.sendspace_adjust_max: Maximum adjusted send buffer size
net.inet.tcp.sendspace_adjust_add: Additional bytes above the active send window

http://nibble.hp48.org/dragonfly/sendspace-adjust.diff