Add uprobe strlen histogram example

[drzaeus77]

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 [email protected]