add an option to maildir handler to enable you to specify a particular

add an option to maildir handler to enable you to specify a particular
ssh identity file, defaulting to the expected /root/.ssh/id_rsa. This
is important to have if you are trying to backup to a different user
on a backuphost than other handlers are using. For example, if the
rdiff handler is connecting to backalbatross@backupserver-pn using
/root/.ssh/id_rsa for public key authentication, and you want to
configure the maildir handler to connect as user
backmaildir@backupserver-pn you will might want to specify a different
identity file for that authentication (if you do not want to share the
id file between the 'backalbatross' and 'backmaildir' users).

Note: this cannot be accomplished with a ssh config file because that
file is host based, ie. you can only specify one user per host
specified in the config

git-svn-id: http://code.autistici.org/svn/backupninja/trunk@563 758a04ac-41e6-0310-8a23-8373a73cc35d