Apply self-imposed path length also to root CAs

Also, some readers of the code find starting the count at 1 for EE
cert confusing (since RFC5280 counts only non-self-issued intermediate
CAs, but we also counted the leaf).  Therefore, never count the EE
cert, and adjust the path length comparison accordinly.  This may
be more clear to the reader.

Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit dc5831da)