[DOCS] start-up script for OpenBSD

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.5/static/index.html
Description:

Hi.

I have just installed OpenBSD 6.0 with the postgresql package (pg 9.5.3)

Section 17.3 of the postgresql documentation gives a recommended startup
script for OpenBSD to be put into /etc/rc.local

The path is incorrect for pg_ctl and postgres. These are located in
/usr/local/bin and not /usr/local/pgsql/bin as given. Naturally this does
not allow postgresql to start at boot.

Also I used '-l logfile' which will write into the postgres user home
directory as the /var/postgresql directory shown, although present, would
have to have permissions or ownership changed to allow a write (I did not
pursue that but dmesg clearly shows an error).

Else the documentation is as always a great help.

Ian