Reorder pieces of header in perl scripts. The desired order is - Prologue part 1 (2 lines with #!) - Program short description - Copyright and license notice - Written-by notice - Program short description (optional) - Program long description (optional) - Prologue part 2 - Time stamp - Code Reported by Paul Eggert. * build-aux/announce-gen: Reorder header. * build-aux/gitlog-to-changelog: Likewise. * build-aux/useless-if-before-free: Likewise. * build-aux/prefix-gnulib-mk: Add copyright notice and short description. * build-aux/update-copyright: Likewise. Add short description. Bump time-stamp-line-limit to 200.