unshare: fix busyloop and reduce racing probability

unshare: fix busyloop and reduce racing probability

Replace busy-loop with waiting on pipe from parent.

Note: reduces racing probability, but still there are window where
it is possible (if parent unshare process will be [externally] killed
between successful read(fds[0]) and mount() calls).

[kzak@redhat.com: - use all-io.h to avoid loops around write() and read(),
                  - use less generic 0x06 byte to sync parent and child]

Signed-off-by: Yuriy M. Kaminskiy <yumkam@gmail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>