lib/core: Add a "break hardlink" API

This imports the code from rpm-ostree:
https://github.com/projectatomic/rpm-ostree/blob/9ff9f6c997d914cb7d97d6b59d8045ba64a1882c/src/libpriv/rpmostree-util.c#L742

I plan to use this for rofiles-fuse to implement
copyup: https://github.com/ostreedev/ostree/issues/1377

But it's just obviously generally useful for projects using
libostree I think.

Closes: #1378
Approved by: jlebon

src/libostree/libostree-devel.sym

@@ -22,6 +22,7 @@
 LIBOSTREE_2017.15 {
   ostree_repo_fsck_object;
   ostree_repo_mark_commit_partial;
+  ostree_break_hardlink;
 } LIBOSTREE_2017.14;
 
 /* Stub section for the stable release *after* this development one; don't