tcp: add sysctl to allow/disallow TSO during SACK loss recovery

[rscheff]

Summary

In preparation for the planned changes around making it possible
for TSO to remain operational during SACk loss recovery, add
a sysctl to retain the current behavior

'''net.inet.tcp.sack.tso = 0'''

Once the related changes are committed, the plan is to switch this
in main to "1" - and either retain this for the next release branch,
or revert the setting to 0 depending on feedback.

Note: Prior to D43355 and D43470, the SACK loss recovery code in
tcp_output is broken and would transmit excessively when TSO is
active.