BUG #14814: Documentation errors for OpenBSD

The following bug has been logged on the website:

Bug reference: 14814
Logged by: Paul Covello
Email address: pfcovello(at)gmail(dot)com
PostgreSQL version: 9.6.2
Operating system: OpenBSD 6.1
Description:

In section 18.3. Starting the Database Server there are instructions for
starting the server on OpenBSD:

if [ -x /usr/local/pgsql/bin/pg_ctl -a -x /usr/local/pgsql/bin/postgres ];
then
su -l postgres -c '/usr/local/pgsql/bin/pg_ctl start -s -l
/var/postgresql/log -D /usr/local/pgsql/data'
echo -n ' postgresql'
fi

However, there are several problems with this instruction. First, the
postgres user on openbsd is _postgresql (all daemon accounts begin with an
underscore).

Second, the path to the pg_ctl and postgres files are wrong.

Finally, the part where the script sets where the log file and database are
do not match with the readme that comes in the package, causing postgress to
error out.

Here is a correct copy of the rc.local file that correctly starts the
postgres server at boot:

if [ -x /usr/local/bin/pg_ctl -a -x /usr/local/bin/postgres ]; then
su -l _postgresql -c '/usr/local/bin/pg_ctl start -s -l
/var/postgresql/logfile -D /var/postgresql/data'
echo -n ' postgresql'
fi

Please update the documentation as I hope this will save users like myself
considerable time troubleshooting this issue as the documentation was
incorrect.

Thank you!

-- Paul Covello.