examples/: Use actual path to Guile interpreter in the shebang.

* examples/ssshd.scm: Rename to `examples/ssshd.scm.in'.
* examples/sssh.scm: Rename to `examples/sssh.scm.in'.
* examples/echo/client.scm: Rename to
  `examples/echo/client.scm.in'.
* examples/echo/server.scm: Rename to
  `examples/echo/server.scm.in'.
* examples/Makefile.am: Substitute `@GUILE@' with the actual path
  to Guile interpreter in *.in files.
* NEWS: Update.