argdist, trace: Support naked executable names in probes (iovisor#720)

Fixes the error message from `BPF._find_exe` which would
occur if argdist or trace had a naked executable name
not qualified with a path, such as:

```
trace 'r:bash:readline "%s", retval'
```

This is now supported again.

src/python/bcc/__init__.py

@@ -120,9 +120,9 @@ def _find_file(filename):
  return filename
 
  @staticmethod
- def _find_exe(cls, bin_path):
+ def find_exe(bin_path):
  """
- _find_exe(bin_path)
+ find_exe(bin_path)
 
  Traverses the PATH environment variable, looking for the first
  directory that contains an executable file named bin_path, and

tools/argdist.py

@@ -379,7 +379,7 @@ def generate_text(self):
  def _attach_u(self):
  libpath = BPF.find_library(self.library)
  if libpath is None:
- libpath = BPF._find_exe(self.library)
+ libpath = BPF.find_exe(self.library)
  if libpath is None or len(libpath) == 0:
  self._bail("unable to find library %s" % self.library)
 

tools/trace.py

@@ -472,7 +472,7 @@ def _attach_u(self, bpf):
  libpath = BPF.find_library(self.library)
  if libpath is None:
  # This might be an executable (e.g. 'bash')
- libpath = BPF._find_exe(self.library)
+ libpath = BPF.find_exe(self.library)
  if libpath is None or len(libpath) == 0:
  self._bail("unable to find library %s" % self.library)