tools: shmsnoop, sofdsnoop handle keyboard interrupt (#2126)

Handle keyboard interrupt to avoid backtrace printed out when user
presses Ctrl-C. Other tools have been fixed recently. Let's fix these
remaining tools.

tools/shmsnoop.py

@@ -316,4 +316,7 @@ def print_event(cpu, data, size):
 b["events"].open_perf_buffer(print_event, page_cnt=64)
 start_time = datetime.now()
 while not args.duration or datetime.now() - start_time < args.duration:
+    try:
         b.perf_buffer_poll(timeout=1000)
+    except KeyboardInterrupt:
+        exit()

tools/sofdsnoop.py

@@ -342,4 +342,7 @@ def print_event(cpu, data, size):
 b["events"].open_perf_buffer(print_event, page_cnt=64)
 start_time = datetime.now()
 while not args.duration or datetime.now() - start_time < args.duration:
+    try:
         b.perf_buffer_poll(timeout=1000)
+    except KeyboardInterrupt:
+        exit()