Only require necessary fields in Special:ResetPass

Only require necessary fields in Special:ResetPass

This fixes r54567.  That made the password fields on Special:ResetPass
always required, but in fact the current password should never be
required (existing users always might have empty passwords), and the new
password is only required if $wgMinimalPasswordLength > 0.

This commit also permits passing array( 'required' ) to
Html::(rawE|e)lement() instead of array( 'required' => 'meaningless' ),
for boolean attribs only.  This syntax is used in SpecialResetpass.