www/uwsgi: Further rc script security improvements

This update introduces a dedicated user for uwsgi and introduces the
uwsgi_socket_owner setting which by default is set to www:www. The
previous change to socket mode of 600 has been modified to 660 as well.

This change further increases security while restoring compatibility.

MFH: 2017Q1
Differential Revision: https://reviews.freebsd.org/D9398