Allow users to optionally specify their envelope sender.

Allow users to optionally specify their envelope sender.

If your normal user is not the same user you are subscribed to a list with,
then the default envelope sender used will cause your messages to bounce or
silently vanish into the ether.

This patch provides an optional parameter to set the envelope sender.
To use it with the sendmail binary, you must have privileges to use the -f
parameter!

Signed-off-by: Robin H. Johnson <redacted>
Signed-off-by: Junio C Hamano <redacted>