make several program to use "chown" promise.

make several program to use "chown" promise.

it allows chown(2) call to change the user or group on a file.

- usr.bin/compress : aka gzip
- usr.bin/mg : open a file for writing
- usr.bin/sed : inplace editing
- usr.bin/sort : if outfile equals one of the input files

ok deraadt@ tb@

(and a reminder from Remi Locherer)

warning: in order to use it, you must have a recent kernel with the new
promise.