Bug 16749: Go with a boolean flag, we love booleans

Bug 16749: Go with a boolean flag, we love booleans

As requested by Jonathan on comment 10 on Bugzilla and supported by Tomas,
we should use the dev_install entry in koha-conf as a boolean flag.
Since we already used it as a path, this patch is not too strict about it.
If the entry is not empty and not equal to "0", we will interpret it as
a true value. The path is taken from the intranetdir entry.

Test plan:
[1] Copy debian/scripts/koha-functions.sh to /usr/share/koha/bin
[2] For a dev install:
    Remove the dev install line, or toggle its value between empty string,
    0 or 1 and each time test stop/start koha-indexer.
    Check the path to rebuild_zebra with ps aux|grep indexer.
    If you have no entry, an empty entry or a zero, you should see a regular
    path.
    (Note: You can do something similar with koha-start-sip.)
[3] For a regular install:
    Remove the dev install line.
    Stop/start koha-indexer or koha-plack, and verify that it still works.
    Add a dev_install line with 0, and repeat stop/start.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
Tested in a package installation of master+16749

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>