urlapi: fix portnumber parsing for ipv6 zone index

An IPv6 URL which contains a zone index includes a '%%25<zode id>'
string before the ending ']' bracket. The parsing logic wasn't set
up to cope with the zone index however, resulting in a malformed url
error being returned. Fix by breaking the parsing into two stages
to correctly handle the zone index.

Closes #3355
Closes #3319
Reported-by: tonystz on Github
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Reviewed-by: Marcel Raad <Marcel.Raad@teamviewer.com>