`mq-sendmail` should use the same exit codes like `sendmail`

[FelixSchwarz]

I am actually not 100% sure which exit codes are used by sendmail. msmtp claims full compatibility so maybe I need to read their source code.

Maybe only use compatible exit codes if called as sendmail (not mq-sendmail) if we want detailed error reporting?

From the Exim command line specification:

• -oem: the return code is 2 if the problem is that the original message has no recipients, or 1 for any other error.

[FelixSchwarz]

Exit code 2 implemented in cd9b684.