lib/deploy: Use a FIFREEZE/FITHAW cycle for /boot

See: http://marc.info/?l=linux-fsdevel&m=149520244919284&w=2

XFS doesn't flush the journal on `syncfs()`. GRUB doesn't know how to follow the
XFS journal, so if the filesystem is in a dirty state (possible with xfs
`/boot`, extremely likely with `/`, if the journaled data includes content for
`/boot`, the system may be unbootable if a system crash occurs.

Fix this by doing a `FIFREEZE`+`FITHAW` cycle.  Now, most people
probably would have replaced the `syncfs()` invocation with those two
ioctls.  But this would have become (I believe) the *only* place in
libostree where we weren't safe against interruption.  The failure
mode would be ugly; nothing else would be able to write to the filesystem
until manual intervention.

The real fix here I think is to land an atomic `FIFREEZETHAW` ioctl
in the kernel.  I might try a patch.

In the meantime though, let's jump through some hoops and set up
a "watchdog" child process that acts as a fallback unfreezer.

Closes: https://github.com/ostreedev/ostree/issues/876

Closes: #1049
Approved by: jlebon

src/libostree/ostree-sysroot-private.h

@@ -33,7 +33,8 @@ typedef enum {
   OSTREE_SYSROOT_DEBUG_MUTABLE_DEPLOYMENTS = 1 << 0,
   /* See https://github.com/ostreedev/ostree/pull/759 */
   OSTREE_SYSROOT_DEBUG_NO_XATTRS = 1 << 1,
-
+  /* https://github.com/ostreedev/ostree/pull/1049 */
+  OSTREE_SYSROOT_DEBUG_TEST_FIFREEZE = 1 << 2,
 } OstreeSysrootDebugFlags;
 
 /**