Drop PRIV_SESS, as agreed on VDD14Q4.
Make the VRTPRIV_* interface slightly more generic to let us use it without needing a req/sess pointer.
Reintroduce PRIV_REQ as PRIV_TASK. PRIV_TASK is available (as separate priv pointers) both in the client side and the backend side VCL functions.
White space fix
White space polish
Merge branch 'vmodpriv'
Handle default values without spaces around the equal sign
Export the VRT runtime version.

The vmod packages need this to know if they are
installable or not for this Varnish version.
Rework example

Fixes #1636
Extract gitref using same method as debs.
Make vrt_selecthttp public
Reduce fetch_chunksize to 16k as agreed on VDD14Q4.
Sort options
Update examples
Restore the default SIGSEGV handler during pan_ic

Leaving it enabled could hide panics.

Fixes #1639
Check pointers before dereferencing - avoid SIGSEGV while panicking
Initialize bo->vfc along with the rest of the busyobj so panics
work.  (slink's pointer test for bo->vfc is impotent because its bo->vfc[1])