support for pwgen;

--- a/src/Pure/System/linux.scala	Tue Dec 10 01:06:39 2019 +0100
+++ b/src/Pure/System/linux.scala	Tue Dec 10 16:45:56 2019 +0100
@@ -12,6 +12,11 @@
 
 object Linux
 {
+  /* required packages */
+
+  val packages: List[String] = List("pwgen")
+
+
   /* check system */
 
   def check_system(): Unit =
@@ -138,4 +143,13 @@
     service_enable(name)
     service_restart(name)
   }
+
+
+  /* passwords */
+
+  def generate_password(length: Int = 10): String =
+  {
+    require(length >= 6)
+    Isabelle_System.bash("pwgen " + length + " 1").check.out
+  }
 }

--- a/src/Pure/Tools/phabricator.scala	Tue Dec 10 01:06:39 2019 +0100
+++ b/src/Pure/Tools/phabricator.scala	Tue Dec 10 16:45:56 2019 +0100
@@ -21,7 +21,7 @@
   /* required packages */
 
   val packages: List[String] =
-    Build_Docker.packages :::
+    Build_Docker.packages ::: Linux.packages :::
     List(
       // https://secure.phabricator.com/source/phabricator/browse/master/scripts/install/install_ubuntu.sh 15e6e2adea61
       "git", "mysql-server", "apache2", "libapache2-mod-php", "php", "php-mysql",