if: support pagespeed directives in location if blocks

This adds support for putting a `pagespeed` directive inside an `if` block inside a `location` block.  These were previously disallowed because we didn't set `NGX_HTTP_LIF_CONF`.

This also fixes the segfault in #640 which was due to not understanding that a `location` block could have another `location` block as a parent (which happens when using `if` inside `location`).

This does not add support for `pagespeed` directives inside `if` blocks that sit immediately in a `server` block.  With `NGX_HTTP_SIF_CONF` enabled those directives would act as if the `if` block didn't exist, applying directly to the server scope.  I feel like I'm missing something obvious, but I don't see why this is happening.  Regardless, I've left `NGX_HTTP_SIF_CONF` off for now.