Add support for old PostgreSQL binary directories used by Arch Linux

Summary:
Arch Linux packages places old PostgreSQL in `/opt/pgsql-PG_VERSION/`.
See https://www.archlinux.org/packages/extra/x86_64/postgresql-old-upgrade/files/ .

Contributed by nl6720 <nl6720@gmail.com>.

Test Plan:
= The lazy method =

Install [[https://www.archlinux.org/packages/extra/x86_64/postgresql/|postgresql]] and [[https://www.archlinux.org/packages/extra/x86_64/postgresql-old-upgrade/|postgresql-old-upgrade]].
Create `~/.config/akonadi/akonadiserverrc`:

```
[%General]
Driver=QPSQL

[QPSQL]
ServerPath=/opt/pgsql-10/bin/pg_ctl
InitDbPath=/opt/pgsql-10/bin/initdb
```

Start Akonadi: `akonadictl start`.
Stop Akonadi: `akonadictl stop`.

Update `~/.config/akonadi/akonadiserverrc`:

```
[%General]
Driver=QPSQL

[QPSQL]
...
ServerPath=/usr/bin/pg_ctl
InitDbPath=/usr/bin/initdb
...
```
Start Akonadi: `akonadictl start`.
The database cluster should get successfully updated.

Reviewers: #kde_pim, dvratil

Reviewed By: #kde_pim, dvratil

Subscribers: dvratil, kde-pim, #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D22154