Modules: extracted config-time merging into separate function.

Modules: extracted config-time merging into separate function.

This introduces ngx_js_merge_conftime_loc_conf() to handle merging
of configuration-time properties.

Normally ngx_js_merge_conf() does all the default value initialization
for child location configurations.

There is a special case for global "http" or "stream" configuration
where the parent configuration needs to be initialized (so it can be
reused by server configurations if no additional directives were
defined in them). But parent configurations are not initialized by
ngx_js_merge_conf().

Most of the ngx_js_loc_conf_t values are only used at runtime, so
only configuration-time values need to be merged in the parent.
The runtime values will be provided from the appropriate
ngx_js_loc_conf_t during request processing.

Previously, configuration-time merging was done inline. Extracting
it into a dedicated function simplifies adding new configuration-time
properties.