#16611: BaseCookie now parses 'secure' and 'httponly' flags.

Previously it generated them if they were given a value, but completely
ignored them if they were present in the string passed in to be parsed.  Now
if the flag appears on a cookie, the corresponding Morsel key will reference a
True value.  Other pre-existing behavior is retained in this maintenance
patch: if the source contains something like 'secure=foo', morsel['secure']
will return 'foo'.  Since such a value doesn't round trip and never did (and
would be a surprising occurrence) a subsequent non-bug-fix patch may change
this behavior.

Inspired by a patch from Julien Phalip, who reviewed this one.