The keys for setting the username and password for incoming and outgoing
mail are inconsisent: one uses `USERNAME` and `PASSWORD`, the other uses
`USER` and `PASSWD`.
To make things simpler, allow both to be configured by either, thus,
`[mailer].USERNAME` and `[mailer.PASSWORD]` will be aliases for `.USER`
and `.PASSWD`, and similarly, `[email.incoming].USER` and
`[email.incoming].PASSWD` will be aliases for `.USERNAME` and
`.PASSWORD`.
Fixes#3355.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
