GNU sync implementation

1) GNU coreutils **sync** implemented.
2) **UNIX_PROGS** in Makefile sorted in alphabetical order.

N.B.: I have a draft implementation on **sync** for Windows which is based on **Sysinternals Sync** by Mark Russinovich and its **flushvol** analogue by Eugene Muzychenko. However, that draft implementation heavily depends on **kernel32** library; moreover, it requires administrative privileges (UNIX/Linux systems don't require any CAP_SYS_* for issuing **sync()** syscall). Let me know if when we Windows implementation.