unistd: introduce nice()

This commit implements nice() as specified in POSIX.1-2008.

Fixes #4932.

Change-Id: Ibd8d4636d9e3a8adf0f26a081d5b1180f0cbb839
Reviewed-on: https://review.haiku-os.org/c/863
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>