Tools: properly handle the keyboard interrupt (#2083)

Many tools rely on the user to type Ctrl-C to end, but don't actually
catch the keyboard interrupt and thus show an ugly backtrace when it
happens. Let's catch the interrupt.
diff --git a/tools/dbslower.py b/tools/dbslower.py
index 92d4127..24e6394 100755
--- a/tools/dbslower.py
+++ b/tools/dbslower.py
@@ -230,4 +230,7 @@
 
 bpf["events"].open_perf_buffer(print_event, page_cnt=64)
 while True:
-    bpf.perf_buffer_poll()
+    try:
+        bpf.perf_buffer_poll()
+    except KeyboardInterrupt:
+        exit()