implement per-object patching/relocations

The recent module patching code has exposed some problems with our data
structures. We currently patch the funcs and dynrelas individually,
which is kind of scary now that different objects can be patched at
different times.  Instead it's cleaner and safer to group them by
patched object.

This patch implements per-object patching and relocations by refactoring
the interfaces:

- Completely separate the create-diff-object <-> patch module interface
  from the patch module <-> core module interface. create-diff-object
  will include "kpatch-patch.h" but not "kpatch.h". Thus,
  create-diff-object has no knowledge about the core module's
  interfaces, and the core module has no knowledge about the patch
  module's special sections.

- Newly added kpatch-patch.h defines the format of the patch module
  special sections. It's used by create-diff-object to create the
  special sections and used by the patch module to read them.

- kpatch.h still defines the core module interfaces. Each kpatch_module
  has a list of kpatch_objects for each module object to be patched.
  Each kpatch_object has a list of kpatch_funcs and a list of
  kpatch_dynrelas. The patch module creates these lists when populating
  kpatch_module.

This way of structuring the data allows us to patch funcs and dynrelas
on a per patched object basis, which will allow us to catch more error
scenarios and make the code easier to manage going forward.  It also
allows the use of much more common code between kpatch_register() and
kpatch_module_notify().