libmount: use memmove when overlap possible

libmount: use memmove when overlap possible

When unmounting some mountpoints as an unprivileged user (via the
'user' option in fstab), the umount fails. Debug output of 'umount
/opt' reveals:

17760: libmount: CXT: [0x22890e0]: do umount
17760: libmount: UTILS: moving to /opt parent
17760: libmount: CXT: current directory moved to / [last_component='opt']
17760: libmount: CXT: [0x22890e0]: umount(2) [target='pt', flags=0x00000000]

valgrind shows the problem:

==23544== Source and destination overlap in memcpy(0x58d1370, 0x58d1371, 4)
==23544==    at 0x4C2BBC3: memcpy@@GLIBC_2.14 (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==23544==    by 0x4E537C3: mnt_chdir_to_parent (utils.c:168)
==23544==    by 0x4E45E4C: mnt_context_do_umount (context_umount.c:601)
==23544==    by 0x4E46513: mnt_context_umount (context_umount.c:855)
==23544==    by 0x403476: umount_one (umount.c:299)
==23544==    by 0x402B34: main (umount.c:629)
==23544==

ref: https://bugs.archlinux.org/task/36968
Signed-off-by: Dave Reisner <dreisner@archlinux.org>