zed: support subject as header in zed_notify_email()

Some minimal MUAs don't support passing the subjects as cmdline option.
This commit checks if "@SUBJECT@" is missing in ZED_EMAIL_OPTS and then
prepends a subject header to the notification message.
Also set a default for ${subject}.

Reviewed-by: Ahelenia Ziemia<C5><84>ska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Daniel Hiepler <d-git@coderdu.de>
Closes #13440