sed(1): Sync with FreeBSD.

sed(1): Sync with FreeBSD.

* Add workaround for a back reference when no corresponding parenthesized
  subexpression is defined. For example, the following command line caused
  unexpected behavior like segmentation fault:

  % echo test | sed -e 's/test/\1/'

* Fix the code to conform to the "or more" part of the following POSIX
  specification.

  "A function can be preceded by one or more '!' characters, in which
  case the function shall be applied if the addresses do not select
  the pattern space."

* Implement "addr1,+N" ranges - not dissimilar to grep's -A switch.