MEDIUM: stats: add the ability to kill sessions from the admin interface

It was not possible to kill remaining sessions from the admin interface,
which is annoying especially when switching to maintenance mode. Now it's
possible.
diff --git a/src/dumpstats.c b/src/dumpstats.c
index a01c6ed..d79c880 100644
--- a/src/dumpstats.c
+++ b/src/dumpstats.c
@@ -3173,6 +3173,7 @@
 					"<option value=\"enable\">Enable</option>"
 					"<option value=\"stop\">Soft Stop</option>"
 					"<option value=\"start\">Soft Start</option>"
+					"<option value=\"shutdown\">Kill Sessions</option>"
 					"</select>"
 					"<input type=\"hidden\" name=\"b\" value=\"#%d\">"
 					"&nbsp;<input type=\"submit\" value=\"Apply\">"