Variables: honor no_cacheable for not_found variables.

Variables: honor no_cacheable for not_found variables.

Variables with the "not_found" flag set follow the same rules as ones with
the "valid" flag set.  Make sure ngx_http_get_flushed_variable() will flush
non-cacheable variables with the "not_found" flag set.

This fixes at least one known problem with $args not available in a subrequest
(with args) when there were no args in the main request and $args variable was
queried in the main request (reported by Laurence Rowe aka elro on irc).

Also this eliminates unneeded call to ngx_http_get_indexed_variable() in
cacheable case (as it will return cached value anyway).