Move this test into the pen.

Spotted by:	fgs
Start a new transaction (xid) on restart
Give SLT_Begin and SLT_Link a third field "why", and normalise the
first field to the type of entity.

Martin will commit corresponding and necessary changes to the
API in a matter of milliseconds or possibly hours or days, so
minor weirdness from varnishlog/ncsa can be expected for a brief
amount of time.

Testcase l00000 & l00001 are also broken until the API catches up.
Quench a warning about empty format args.
Move the probing of system default tcp-keepalive parameters
to a separate source file and open (and close) a socket specifically
for that purpose, so we can do the right thing with parameter
defaults.
Keep Makefile.phk prototype up to date