Added support for DragonFly BSD Mail Agent (dma)

[felixjogris]

General information

Add support for mailq output if DragonFly BSD Mail Agent (dma) is installed on FreeBSD. Starting FreeBSD 14, dma became the default MTA.

Proposed changes

Output no more than 7 lines (=2 queued mails) of dma's mailq output. As dma outputs no summarizing status line, construct one if there is at least one queued mail.