urlapi: add CURLU_GUESS_SCHEME and fix hostname acceptance

In order for this API to fully work for libcurl itself, it now offers a
CURLU_GUESS_SCHEME flag that makes it "guess" scheme based on the host
name prefix just like libcurl always did. If there's no known prefix, it
will guess "http://".

Separately, it relaxes the check of the host name so that IDN host names
can be passed in as well.

Both these changes are necessary for libcurl itself to use this API.

Assisted-by: Daniel Gustafsson
Closes #3018