Add function which implements the bash 'which' feature.

* Add function _which(), which is shamelessly stolen from Twisted's
   twisted.python.procutils module.

gnupg.py

@@ -858,6 +858,44 @@ def _sanitise(*args, **kwargs):
 
     return sanitised
 
+def _which(executable, flags=os.X_OK):
+    """Shamelessly pilfered from Twisted's :mod:`twisted.python.proutils`.
+
+    Search PATH for executable files with the given name.
+
+    On newer versions of MS-Windows, the PATHEXT environment variable will be
+    set to the list of file extensions for files considered executable. This
+    will normally include things like ".EXE". This fuction will also find files
+    with the given name ending with any of these extensions.
+
+    On MS-Windows the only flag that has any meaning is os.F_OK. Any other
+    flags will be ignored.
+
+    @type name: C{str}
+    @param name: The name for which to search.
+
+    @type flags: C{int}
+    @param flags: Arguments to L{os.access}.
+
+    @rtype: C{list}
+    @param: A list of the full paths to files found, in the order in which
+            they were found.
+    """
+    result = []
+    exts = filter(None, os.environ.get('PATHEXT', '').split(os.pathsep))
+    path = os.environ.get('PATH', None)
+    if path is None:
+        return []
+    for p in os.environ.get('PATH', '').split(os.pathsep):
+        p = os.path.join(p, name)
+        if os.access(p, flags):
+            result.append(p)
+        for e in exts:
+            pext = p + e
+            if os.access(pext, flags):
+                result.append(pext)
+    return result
+
 class GPG(object):
     """Encapsulate access to the gpg executable"""
     decode_errors = 'strict'