Now that patch #642500 is in, it only seems logical to
also allow keyword args in dict.update(). This patch
simply reuses dict_init(), so d.update(seq_of_items)
also works. I'm not conviced of the neccessity of this
patch, but the implementation is very simple. Doc and
test will follow (also: gladly accepted ;-) if the idea
is accepted.

Side effect: d.update() (no args!) raises no exception
anymore.