Make `PGSSL` config variable work setting it to the value of the variable, rather than as a toggle flag

**Brief Description**
The `PGSSL` variable is listed in the example local config as:

`PGSSL=true`

which implies it is a variable that can be turned off by changing the `true` to `false`.

In fact, however, `PGSSL` is off by default, and it is turned on if the `PGSSL` variable is set to any value, including `false`. 

This is confusing and is different to how all other variables work.