Provide a sysctl to toggle reporting of TCP debug logging:

sys.net.inet.tcp.log_debug = 1

It defaults to enabled for the moment and is to be turned off for
the next release like other diagnostics from development branches.

It is important to note that sysctl sys.net.inet.tcp.log_in_vain
uses the same logging function as log_debug. Enabling of the former
also causes the latter to engage, but not vice versa.

Use consistent terminology in tcp log messages:

"ignored" means a segment contains invalid flags/information and

is dropped without changing state or issuing a reply.

"rejected" means a segments contains invalid flags/information but

is causing a reply (usually RST) and may cause a state change.

Approved by: re (rwatson)