Reject config with unknown directives before committing to it (#1897)

Reject config with unknown directives before committing to it (#1897)

Ideally, we want to reject configurations with unknown directives before
applying any configuration changes that correspond to known directives,
but current apply-as-you-parse architecture makes that impractical.
Pending smooth reconfiguration refactoring will make that possible, but
we can make a step towards that ideal future now.

Rejecting bad configurations before calling configDoConfigure() reduces
the set of configuration errors that Squid can detect in one execution
(because configDoConfigure() error-checking code is not reached), but
that small reduction is a lesser evil compared to running
configDoConfigure() with a clearly broken config, especially when we are
going to kill Squid anyway. While many legacy parse_foo() functions do
apply significant changes before configDoConfigure(), we cannot easily
prevent that (for now). We can easily prevent configDoConfigure().