port knetattach from kurl to qurl

Summary:
kurl is deprecated.

- s/kurl/qurl
- non-existent functions ported per deprecation recommendation
- fileEncoding replaced with url.queryItemValue("charset") which should be
  the fitting replacement from a quick look at the kurl code

unfortunately there are still more kdelibs4support requirements left it
seems :(

Test Plan:
- builds
- connects to smb

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16273