Support setting the email sender

Support setting the email sender

Bounce emails are sent to the Return-Path: address.  The Return-Path:
address can be set by specifying the SMTP 'MAIL FROM:' address.  The
SMTP 'MAIL FROM:' address can be set by passing the -f option to
sendmail.

Add a new $Girocco::Config::sender setting to allow the value passed
to $Girocco::Config::sendmail_bin via the -f option to be specified
and by default set it to the value of $Girocco::Config:admin, but
allow it to be unset to suppress passing the -f option at all.

The mail.sh script will now ignore any hooks.envelopesender setting
in favor of the new $Girocco::Config::sender setting.