Be able to detect parse error during processing of backend Vary
headers, and do 503 on parse error.

Conflicts:
	bin/varnishd/cache.h
	bin/varnishd/cache/cache_req_fsm.c
	bin/varnishd/cache_vary.c
Don't panic on malformed Vary headers.

Fixes: #1275

Test case by: Dag Haavi Finstad
Return 503 when Vary-headers references header names more than 127
(out limit) characters long.

Fixes: #1274

Test case by: Dag Haavi Finstad
Throw 503 on exceeding max 65k Vary header constituent limit.
Remove dodgy workaround introduced for #763.

With fix for #1275 we can now do a proper 503 when malformed
Vary-headers are received from backend. Error out instead of accepting
bogus ':'.

Conflicts:
	bin/varnishd/cache_vary.c
Document Vary parse error behavior