add update cli-command; add update-channel setting (stable|testing)

Signed-off-by: Michael Kaufmann <d00p@froxlor.org>

install/froxlor.sql.php

@@ -699,6 +699,7 @@ opcache.validate_timestamps'),
 	('system', 'froxlorusergroup_gid', ''),
 	('system', 'acmeshpath', '/root/.acme.sh/acme.sh'),
 	('system', 'distribution', ''),
+	('system', 'update_channel', 'stable'),
 	('api', 'enabled', '0'),
 	('2fa', 'enabled', '1'),
 	('panel', 'decimal_places', '4'),