Support for .forward files

[ghost]

Noticed dma doesn't support .forward files and there's a placeholder in dma.c

[emaste]

@corecode and I have had some offline discussion about this. The suggestion is to add a small suid standalone helper program e.g. dma-read-forward that emits the desired forward entry on stdout.

Links to some commentary on .forward files:

• https://docstore.mik.ua/orelly/networking/sendmail/ch25_07.htm
• https://docs.oracle.com/cd/E19120-01/open.solaris/819-1634/mailrefer-39/index.html