passwd: add an unprivileged user for automountd

[kevans]

Summary

This will be used to do some limited privilege dropping while parsing
map files in the next commit.

[markj]

Is there a reason not to use "nobody"?

[kevans]

In D56679#1298826, @markj wrote:

Is there a reason not to use "nobody"?

Hmm, I didn't really capture it in the commit message. My theory was that there's a reasonable chance that someone may want to grant automountd some permissions (e.g., maybe they want it to be able to read /dev disk nodes)- the associated patch tries to preserve its supplementary groups just for that.