implement a command to get the auth password

[rhizoome]

I use dma interactive - non-root, non-cron, only interactive programs calling sendmail, dma -q, etc - so a config like

!mylogin|smtp.host.com:pass show smpt.host.com/mylogin

make sense. Since this is not a too common use-case this patch is either:

• Meant for interested parties who use dma interactive
• Or if there is interest by upstream. It is a prototype.
  • Using "!" as a signifier for a command is probably not perfect, there might be a login starting with "!"
  • Documentation is missing
  • Maybe more details aren't well thought out yet
  • Error handling is solid, I think