HTTP/1.1: honour Cache-Control before Pragma:no-cache

HTTP/1.1: honour Cache-Control before Pragma:no-cache

HTTPbis WG clarifications on the interaction between Cache-Control and
Pragma specify that Cache-Control may be explicitly crafted to allow
conditional caching by HTTP/1.1 caches and Pragma:no-cache supplied to
prevent caching by older HTTP/1.0 caches unable to understand the
Cache-Control properly.

Squid does obey Cache-Control. So we can deprecate Pragma to the side
cases when Cache-Control is absent entirely.