argdist, trace: Support naked executable names in probes (#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.
diff --git a/tools/trace.py b/tools/trace.py
index 696ca61..ee41373 100755
--- a/tools/trace.py
+++ b/tools/trace.py
@@ -472,7 +472,7 @@
                 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)