qt: Add support for flags in LDAP server options

* lang/qt/src/qgpgmenewcryptoconfig.cpp (parseURL): Handle extended
LDAP server option syntax.
(portToString): New.
(splitURL): Append flags to LDAP server option.

This adds support for the extended syntax of LDAP server options
introduced in gpg 2.2.18/2.3. The flags are stored as fragment of a
QUrl.

• GnuPG-bug-id: T5217