Add basic support of translators.

* lazycat-daemon/lazycatd.scm (translate-command): New procedure.
  (exec-cmd-on-host): Add support of translators.  Change arguments.
  All callers updated.
  (exec-cmd): Likewise.
  (handle-exec-req): Add support of translators.
* ui/lc.scm (handle-exec): Handle `translate' option.
  (exec-cmd): Add support of translators.  Change arguments.
  All callers updated.
  (exec-cmd-on-host): Likewise.
  (print-help): Update.
* Makefile.am (SUBDIRS): Add `trans' subdirectory.
* configure.ac: Update.
* common/protocol.scm (*cmd-exec*): Update comment.
* common/translator.scm: New file.
* lazycat-daemon/translator-list.scm: New file.
* trans/portage.scm: New file.
* trans/apt.scm: New file.
* trans/Makefile.am: New file.