www/nginx: Change default log locations to avoid /var/log pollution

The nginx error log default will always be touched regardless of the value
of the error_log directive in nginx.conf. This is not a bug. It also
breaks shell tab completion for the sane default of /var/log/nginx.

This change aligns the compiled-in default and the configuration default
of error log to a new default /var/log/nginx directory.

PR: 195916
Approved by: maintainer timeout (15 months)