Improved check for duplicate path names in ngx_add_path().

The same path names with different "data" context should not be allowed.

In particular it rejects configurations like this:

    proxy_cache_path /var/cache/ keys_zone=one:10m max_size=1g inactive=5m;
    proxy_cache_path /var/cache/ keys_zone=two:20m max_size=4m inactive=30s;