Provide new 'is_exe' function used by 'which'

As noted by bug report http://bugs.python.org/issue14706,

"This is not a Python bug. os.access() is just a wrapper
 around the POSIX access() function"

http://pubs.opengroup.org/onlinepubs/9699919799/functions/faccessat.html

""" If any access permissions are checked, each shall be
    checked individually, as described in XBD File Access
    Permissions, except that where that description refers
    to execute permission for a process with appropriate
    privileges, an implementation may indicate success for
    X_OK even if execute permission is not granted to any
    user.
"""

pexpect/__init__.py

@@ -80,6 +80,7 @@
     import traceback
     import signal
     import codecs
+    import stat
 except ImportError:  # pragma: no cover
     err = sys.exc_info()[1]
     raise ImportError(str(err) + '''
@@ -1957,15 +1958,48 @@ def search(self, buffer, freshlen, searchwindowsize=None):
         return best_index
 
 
+def is_exe(fname):
+    # follow symlinks,
+    fpath = os.path.realpath(fname)
+
+    # return False for non-files (directories, fifo, etc.)
+    if not os.path.isfile(fpath):
+        return False
+
+    # On Solaris, etc., "If the process has appropriate privileges, an
+    # implementation may indicate success for X_OK even if none of the
+    # execute file permission bits are set."
+    #
+    # For this reason, it is necessary to explicitly check st_mode
+
+    # get file mode using os.stat, and check if `other',
+    # that is anybody, may read and execute.
+    mode = os.stat(fpath).st_mode
+    if mode & stat.S_IROTH and mode & stat.S_IXOTH:
+        return True
+
+    # get current user's group ids, and check if `group',
+    # when matching ours, may read and execute.
+    user_gids = os.getgroups() + [os.getgid()]
+    if (os.stat(fpath).st_gid in user_gids and
+            mode & stat.S_IRGRP and mode & stat.S_IXGRP):
+        return True
+
+    # finally, if file owner matches our effective userid,
+    # check if `user', may read and execute.
+    user_gids = os.getgroups() + [os.getgid()]
+    if (os.stat(fpath).st_uid == os.geteuid() and
+            mode & stat.S_IRUSR and mode & stat.S_IXUSR):
+        return True
+
+
 def which(filename):
     '''This takes a given filename; tries to find it in the environment path;
     then checks if it is executable. This returns the full path to the filename
     if found and executable. Otherwise this returns None.'''
 
     # Special case where filename contains an explicit path.
-    if (os.path.dirname(filename) != '' and
-            os.access(filename, os.X_OK) and
-            os.path.isfile(os.path.realpath(filename))):
+    if os.path.dirname(filename) != '' and is_exe(filename):
         return filename
     if 'PATH' not in os.environ or os.environ['PATH'] == '':
         p = os.defpath
@@ -1974,8 +2008,7 @@ def which(filename):
     pathlist = p.split(os.pathsep)
     for path in pathlist:
         ff = os.path.join(path, filename)
-        if (os.access(ff, os.X_OK) and
-                os.path.isfile(os.path.realpath(ff))):
+        if is_exe(ff):
             return ff
     return None