[__opendkim[_genkey]]: potential for wrong permissions with --userid #17

Porting these types to work on FreeBSD I noticed that __opendkim has support for a --userid parameter.

OTOH, __opendkim_genkey just uses chown opendkim:opendkim KEY.private; therefore assuming that the service runs with those credentials.

Without knowing the internals of Alpine, I can imagine this being an issue as per opendkim(8):

   Thus, keys referenced by the KeyTable must always be accessible for
   read by the unprivileged user.  Also, run-time reloads are not possible
   if any of the other files will not be readable by the unprivileged
   user.

Instead of making things complex, I suggest deprecating __opendkim's --userid argument; does that sound ok? Which use-case am I missing?

cc: @sparrowhawk

Uuugh, pretty sure that's a loosely concentrated me trying to ease further generalization of the type without actually thinking it through. We don't use that parameter anywhere, and I don't see why anyone would want to use another user than the one that is preconfigured in the package for their distribution.

Go for deprecation.