Install _all_ headers under ${prefix}/include/..

Fixes #1616
Spelling
Move test to (d)irectors

I'm not sure how valuable this test is but put it in the right place
until it's decided.
Don't mention minimum values twice
Don't panic on incomplete VDP::gunzip, the backend may have
abandonned us.

Fixes	#1624
Turn bereq.uncacheable read-only

This had the same effect as writing to beresp.uncacheble.
Make obj.uncacheable available in vcl_deliver

With this is possible to find out whether the request was a pass (or
hit-for-pass) in a single place.
Add to struct req, incomplete in VCC.
Add per-request send_timeout

This change introduces `req.send_timeout`, which by default is the value
of parameter send_timeout.

It allows for setting timeout from VCL, so that large file downloads or
streams shown in real time can get an extra time allotment without
opening the whole site to attrition attacks.