sysroot: Add an API to lock

If a system administrator happens to type `ostree admin upgrade`
multiple times, currently that will lead to a potentially corrupted
system.

I originally attempted to do locking *internally* in `libostree`, but
that didn't work out because currently a number of the commands
perform multi-step operations that all need to be serialized.  All of
the current code in `ostree admin deploy` is an example.

Therefore, allow callers to perform locking, as most of the higher
level logic is presently implemented there.

At some point, we can revisit having internal locking, but it will be
difficult.  A more likely approach would be similar to Java's approach
with concurrency on iterators - a "fail fast" method.

src/libostree/ostree-sysroot-private.h

@@ -20,6 +20,7 @@
 
 #pragma once
 
+#include "libglnx.h"
 #include "ostree.h"
 #include "ostree-kernel-args.h"
 #include "ostree-bootloader.h"
@@ -31,6 +32,7 @@ struct OstreeSysroot {
 
   GFile *path;
   int sysroot_fd;
+  GLnxLockFile lock;
 
   gboolean loaded;
   
@@ -43,8 +45,11 @@ struct OstreeSysroot {
 
   /* Only access through ostree_sysroot_get_repo() */
   OstreeRepo *repo;
+
 };
 
+#define OSTREE_SYSROOT_LOCKFILE "ostree/lock"
+
 gboolean
 _ostree_sysroot_read_boot_loader_configs (OstreeSysroot *self,
                                           int            bootversion,