Fix #1507: Inconsistent \festival output.

Fix #1507: Inconsistent \festival output.

* input/regression/song-skip-noword.ly

  reset *skip-word* to default at end of file to prevent override leaking
  into other regression tests

* ly/festival.ly (festivalsyl):

  use paramaterize to set *syllabify* and limit its scope locally to the
  music function's body

* scm/song.scm:

  make global configuration variables srfi-39 paramaters