MINOR: http: add a function to check request's cache-control header field

MINOR: http: add a function to check request's cache-control header field

The new function check_request_for_cacheability() is used to check if
a request may be served from the cache, and/or allows the response to
be stored into the cache. For this it checks the cache-control and
pragma header fields, and adjusts the existing TX_CACHEABLE and a new
TX_CACHE_IGNORE flags.

For now, just like its response side counterpart, it only checks the
first value of the header field. These functions should be reworked to
improve their parsers and validate all elements.