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.c

@@ -166,6 +166,7 @@ ostree_sysroot_init (OstreeSysroot *self)
 {
   const GDebugKey keys[] = {
     { "mutable-deployments", OSTREE_SYSROOT_DEBUG_MUTABLE_DEPLOYMENTS },
+    { "test-fifreeze", OSTREE_SYSROOT_DEBUG_TEST_FIFREEZE },
     { "no-xattrs", OSTREE_SYSROOT_DEBUG_NO_XATTRS },
   };