Enforce $wgMinimalPasswordLength client-side

Enforce $wgMinimalPasswordLength client-side

. . . except not really.  It works fine on Opera 9.6, except for the
slight detail that if you enter a password that's too short, Opera will
helpfully repeat your password back to you un-*ed when telling you it's
too short.  Same in Opera 10.00 Beta 3.  So the code is commented out,
and there are no functional changes.  We'll need UA sniffing when the
code is uncommented.  But I already wrote it, so may as well commit it
for future use.

This recycles the "passwordtooshort" message to provide the client-side
error message, using the title attribute on the input.  Since the title
attribute might be displayed when the user hasn't actually entered an
invalid password, I've reworded it to not imply the user actually
entered an incorrect password, so it just states the requirement.  (This
accords with the advice given in the HTML 5 spec.)  I didn't make up a
new message name for that, because it's not a big deal if translations
do imply that the password is wrong, since that should theoretically be
the most common case anyway.