+ Generally test nkeywords against 0 instead of keywords against NULL
  (saves a little work if an empty keywords dict is passed, and is
  conceptually more on-target regardless).
+ When a call erroneously specifies a keyword argument both by position
  and by keyword name:
    - It was easy to provoke this routine into an internal buffer overrun
      by using a long argument name.  Now uses PyErr_format instead (which
      computes a safe buffer size).
    - Improved the error msg.