Unify API for file descriptor sharing

Summary:
Pass socket path as std::string to SocketAddress, FdSender and FdReceiver.
This way FdSender and SocketAddress remain qt-free and FdReceiver doesn't
stand out.

Reviewers: dfaure, ossi

Reviewed By: dfaure, ossi

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12745