kpatch: improve "unload --all" error message

In the case of "kpatch unload --all", just show the usage help instead
of this:

  # kpatch unload --all
  basename: unrecognized option '--all'
  Try 'basename --help' for more information.
  unloading patch module:
  basename: unrecognized option '--all'
  Try 'basename --help' for more information.
  rmmod: ERROR: Module  is builtin.
  kpatch: failed to unload patch --all
kpatch: consistent naming: module vs patch

Try to be more consistent with the naming of patch modules.  They are
patch modules, where patch is the adjective and module is the noun.  So
calling them patches is confusing.
Merge pull request #231 from jpoimboe/unload-all-error

kpatch: a few minor script cleanups