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.