Add chgrep.

Chgrep searches the input files (or standard input if no files are named)
for oldstring and changes them to new string. If some file and file.lock
exist then chgrep leaves both files unchanged. This program can be very use-
full for mail servers. Chgrep is fast, much faster than grep or sed.

PR: 51198
Submitted by: Kirill Ponomarew <ponomarew@oberon.net>