The default user running PostgreSQL 9.6 server has been changed to postgres, see /usr/ports/UPDATING entry from 20160905, however the pkg-message displayed after installation still says
[...]For postmaster settings, see ~pgsql/data/postgresql.conf
NB. FreeBSD's PostgreSQL port logs to syslog by default
See ~pgsql/data/postgresql.conf for more info[...]
~pgsql should be changed to ~postgres twice here.