Tools fixes (#2009)

* tools: the argument of get_kprobe_functions() should be a bytes object

It fixes the following error:
Traceback (most recent call last):
  File "/usr/share/bcc/tools/ext4dist", line 189, in <module>
    if BPF.get_kprobe_functions('ext4_file_read_iter'):
  File "/usr/lib/python3.6/site-packages/bcc/__init__.py", line 519, in get_kprobe_functions
    if (t.lower() in [b't', b'w']) and re.match(event_re, fn) \
  File "/usr/lib64/python3.6/re.py", line 172, in match
    return _compile(pattern, flags).match(string)
TypeError: cannot use a string pattern on a bytes-like object

* tools: fix freeze in offwaketime

Since commit 47cecb65, the sigint signal is ignored
unconditionally which prevents offwaketime to be stopped by
pressing Ctrl-C when run without a specified duration.