ui/lc.scm: Rename some methods.  Improve.

* ui/lc.scm (lazycat-generic-exec): Don't use generic method for
  `exec' handling.  Rename to `handle-exec'.
  (lazycat-add): Rename to `handle-add'.
  (lazycat-rem): Rename to `handle-rem'.
  (lazycat-generic-exec): Rename to `handle-exec'.
  (lazycat-diff): Rename to `handle-diff'.
  (lazycat-set): Rename to `handle-set'.
  (lazycat-get): Rename to `handle-get'.
  (lazycat-list): Rename to `handle-list'.
  (lazycat-stop): Rename to `handle-stop'.
  (main): Update.