Add uprobe strlen histogram example

This example traces all calls to libc's strlen(). The program is attached as a
retprobe, therefore giving access to the resulting string length. The value is
kept in a log2 histogram that is printed to console once per second.

Example:
```
$ sudo ./strlen_hist.py
22:12:51
     strlen return:      : count     distribution
         0 -> 1          : 2041     |****************                        |
         2 -> 3          : 1120     |********                                |
         4 -> 7          : 3300     |**************************              |
         8 -> 15         : 4995     |****************************************|
        16 -> 31         : 2130     |*****************                       |
        32 -> 63         : 562      |****                                    |
^C
```
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

examples/tracing/strlen_hist.py

+#!/usr/bin/python
+
+#
+# strlen_hist.py   Histogram of system-wide strlen return values
+#
+# A basic example of using uprobes along with a histogram to show
+# distributions.
+#
+# Runs until ctrl-c is pressed.
+#
+# Copyright (c) PLUMgrid, Inc.
+# Licensed under the Apache License, Version 2.0 (the "License")
+# 
+# Example output:
+# $ sudo ./strlen_hist.py
+# 22:12:52
+#      strlen return:      : count     distribution
+#          0 -> 1          : 2106     |****************                        |
+#          2 -> 3          : 1172     |*********                               |
+#          4 -> 7          : 3892     |******************************          |
+#          8 -> 15         : 5096     |****************************************|
+#         16 -> 31         : 2201     |*****************                       |
+#         32 -> 63         : 547      |****                                    |
+#         64 -> 127        : 106      |                                        |
+#        128 -> 255        : 13       |                                        |
+#        256 -> 511        : 27       |                                        |
+#        512 -> 1023       : 6        |                                        |
+#       1024 -> 2047       : 10       |                                        |
+# ^C$
+#
+
+from __future__ import print_function
+import bcc
+import time
+
+text = """
+#include <uapi/linux/ptrace.h>
+BPF_HISTOGRAM(dist);
+int count(struct pt_regs *ctx) {
+    dist.increment(bpf_log2l(ctx->ax));
+    return 0;
+}
+"""
+
+b = bcc.BPF(text=text)
+sym="strlen"
+b.attach_uretprobe(name="c", sym=sym, fn_name="count")
+
+dist = b["dist"]
+
+try:
+    while True:
+        time.sleep(1)
+        print("%-8s\n" % time.strftime("%H:%M:%S"), end="")
+        dist.print_log2_hist(sym + " return:")
+        dist.clear()
+
+except KeyboardInterrupt:
+    pass