screencast: Don't leak recorders on failure

ShellRecorder.record() may fail, remove the recorder from the map
in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=737846
screencast: Re-add lockdown support

Commit 81bb7009ea120d3 removed support for the disable-disk-writes
lockdown feature for screencasts, add it back.

https://bugzilla.gnome.org/show_bug.cgi?id=737846
screenshot: Respect lockdown settings

We allow users/admins to lock down disk writes, respect that when
taking screenshots.

https://bugzilla.gnome.org/show_bug.cgi?id=737846