create-diff-object: show object name in log messages

For log_normal and DIFF_FATAL messages, prefix them with the object name
to give more context, which is useful for patches which change multiple
objects.  Also, no need to add the function and line number to
DIFF_FATAL messages, as the error strings already give enough
information.

Example messages:

  meminfo.o: changed function: meminfo_proc_show
  cmdline.o: no changed functions were found
create-diff-object: removed unused function

kpatch_find_changed_functions isn't called by anybody, so remove it.
Merge pull request #130 from jpoimboe/cdo-log-obj-name

create-diff-object logging improvements