installer: avoid bash-specific constructs and switch to /bin/sh

Warning: backwards incompatible change in https://stash.kopano.io/projects/KC/repos/kopanocore/commits/d245d2292aa5505b253ebe684028bb90543d650a

Systems on which /etc/kopano/autorespond was modified manually (for example to messages in cc or less frequent then only every 24 hours) will have received a message during package upgrade that the file was changed, keeping the file as-is will result in a not working auto responder.

The fix is documented at:
https://kb.kopano.io/display/WIKI/Autoresponder+fails+on+Kopano