Add support for FileJob->truncate() in smb/sftp slaves
Summary: This implements the new method added in D26148 in the sftp/smb slaves.
Tested sftp locally with truncate. Seems to work.
Haven't tested locally with smb though.
Reviewers: sitter, dfaure, fvogt
Reviewed By: sitter, dfaure, fvogt
Subscribers: kde-frameworks-devel, kfm-devel, ngraham
Differential Revision: https://phabricator.kde.org/D26191