[users] Introduce userShell setting

- Add a *userShell* key, which can be left out (default, backwards-
   compatible) to retain the old /bin/bash behavior, or explicitly
   set to empty to defer to useradd-configuration, or explicitly
   set to something non-empty to use that shell.

src/modules/users/CreateUserJob.cpp

@@ -147,7 +147,9 @@ CreateUserJob::exec()
     }
 
     QStringList useradd{ "useradd", "-m", "-U" };
-    // TODO: shell-settings
+    QString shell = gs->value( "userShell" ).toString();
+    if ( !shell.isEmpty() )
+        useradd << "-s" << shell;
     useradd << "-c" << m_fullName;
     useradd << m_userName;
 

src/modules/users/UsersViewStep.cpp

@@ -22,9 +22,11 @@
 
 #include "UsersPage.h"
 
+#include "utils/CalamaresUtils.h"
 #include "utils/Logger.h"
-#include "JobQueue.h"
+
 #include "GlobalStorage.h"
+#include "JobQueue.h"
 
 CALAMARES_PLUGIN_FACTORY_DEFINITION( UsersViewStepFactory, registerPlugin<UsersViewStep>(); )
 
@@ -181,5 +183,12 @@ UsersViewStep::setConfigurationMap( const QVariantMap& configurationMap )
             m_widget->addPasswordCheck( i.key(), i.value() );
         }
     }
+
+    QString shell( QLatin1Literal( "/bin/bash" ) );  // as if it's not set at all
+    if ( configurationMap.contains( "userShell" ) )
+        shell = CalamaresUtils::getString( configurationMap, "userShell" );
+        // Now it might be explicitly set to empty, which is ok
+
+    Calamares::JobQueue::instance()->globalStorage()->insert( "userShell", shell );
 }
 

src/modules/users/users.conf

@@ -74,3 +74,12 @@ passwordRequirements:
     libpwquality:
         - minlen=0
         - minclass=0
+
+# Shell to be used for the regular user of the target system.
+# There are three possible kinds of settings:
+#  - unset (i.e. commented out, the default), act as if set to /bin/bash
+#  - empty (explicit), don't pass shell information to useradd at all
+#    and rely on a correct configuration file in /etc/default/useradd
+#  - set, non-empty, use that path as shell. No validation is done
+#    that the shell actually exists or is executable.
+# userShell: /bin/bash