Bring back the "replace" functionality

[jpoimboe]

Currently, when applying an upgraded cumulative patch, where v2 reverts a change done by v1, we have to be careful:

1. compile v1 and note the list of changed functions

2. compile v2 and note the list of changed functions

3. for those functions which changed in v1 and did not change in v2, make a trivial change (i.e., asm("nop") such that kpatch-build will detect the functions as changed, but otherwise their behavior will be the same.

We used to have a "replace" feature which solved this problem. It atomically reverted the previous patch when applying the new one. That feature was deprecated with #557. We should bring it back to kpatch.ko, and we should also port something similar to livepatch.

[jpoimboe]

Soon kpatch.ko will be deprecated in favor of livepatch, and there are already some livepatch patches for this being discussed upstream.