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/nfsslower.py b/tools/nfsslower.py
index 2f92c90..32e91c7 100755
--- a/tools/nfsslower.py
+++ b/tools/nfsslower.py
@@ -325,4 +325,7 @@
 
 b["events"].open_perf_buffer(print_event, page_cnt=64)
 while 1:
+    try:
         b.perf_buffer_poll()
+    except KeyboardInterrupt:
+        exit()