mtree "language" enhancements

[Tim Kientzle]

> On Nov 29, 2015, at 2:49 PM, Tim Kientzle <tim at kientzle.com> wrote:
> 
> Simon also asked:
>> Indeed I'd really like the ability to provide default uid/gid
>> for the case that a uname/gname cannot be looked up.
> 
> I think 'tar' got this right:  If uname and uid are both specified, then look up uname and if that fails, use the specified uid.  Ditto for gname/gid.  In particular, this lets a single specification be used to rebuild a tree on another system with different UIDs or on a system that does not (yet) have a full password file.  An option could be provided for the (rare) case that someone really wants to prefer UIDs to unames.

On further reflection, preferring UIDs to unames would actually be pretty common here.

In particular, NanoBSD (and Crochet and other similar tools) should prefer the UID when building images instead of looking up unames against the build host's password file.

Tim