File ioslave : use Better setting for sendfile syscall

Summary:
Changes :
 * use sendfile when copying file bigger than 2 GB
 * copy 512 kB instead of previously 32 kB for each sendfile or read/write copying iteration

This effectively increase the copying speed, since the filesystem is asked to copy more at once, it lets it reach higher speed before getting the next data to copy.

CCBUG: 342056
CCBUG: 402276

Test Plan: Copying large files or folders with medium sized files (for instance photos) is noticebly faster.

Reviewers: #frameworks, dfaure

Reviewed By: dfaure

Subscribers: bruns, broulik, kde-frameworks-devel

Tags: #frameworks

Maniphest Tasks: T11627

Differential Revision: https://phabricator.kde.org/D28555