CC-3048: Install_minimal should check much sooner if Airtime is already installed

- fixed by creating new php file airtime-installed-check, which checks whether current version
of airtime exists; this file gets called right after root user check and whether debian
package exists check
- moved some code from airtime-install.php to AirtimeInstall.php as static functions
so that we reduce duplicated code
- both the new file airtime-installed-check.php and airtime-install.php use the new
static functions created above

install_minimal/include/airtime-installed-check.php

@@ -0,0 +1,38 @@
+<?php
+/**
+ * @package Airtime
+ * @copyright 2011 Sourcefabric O.P.S.
+ * @license http://www.gnu.org/licenses/gpl.txt
+ *
+ * Checks if a current version of Airtime is installed.
+ * If so, the user is presented with the help menu and can
+ * choose -r to reinstall.
+ */
+require_once(dirname(__FILE__).'/AirtimeInstall.php');
+require_once(__DIR__.'/airtime-constants.php');
+
+AirtimeInstall::ExitIfNotRoot();
+
+$opts = AirtimeInstall::getOpts();
+if ($opts == NULL) {
+    exit(1);
+}
+
+if (isset($opts->h)) {
+    AirtimeInstall::printUsage($opts);
+    exit(1);
+}
+
+$version = AirtimeInstall::GetVersionInstalled();
+// The current version is already installed.
+if (isset($version) && ($version != false) && ($version == AIRTIME_VERSION) && !isset($opts->r)) {
+    echo "Airtime $version is already installed.".PHP_EOL;
+    AirtimeInstall::printUsage($opts);
+    exit(1);
+}
+
+if($version === false){
+    echo "A version of Airtime older than 1.7.0 detected, please upgrade to 1.7.0 first.\n";
+    echo "You will then be able to upgrade to 1.9.0 using this installer.\n";
+    exit(3);
+}