Have qstat request specific attributes

This request has three commits:

- Fix 'qstat -u <username>' which was recently broken when support was added for 'qstat -f -u <username>'
- Request specific attributes from qstat to avoid unnecessary network traffic (changes only in qstat.c)
- Add a new batch request type SelStatAttr to allow status requests specify an attropl and an attrl to both filter and specify which attributes need to be returned.  qstat will fall back to SelStat if SelStatAttr is not supported on the server

In aggregate, this will significantly reduce the amount of data over the wire, especially on large systems (since the exec_host list can be a couple MB).

I'm very comfortable with the first commit and fairly comfortable with the second.  The third is a little outside of my comfort zone, so please review it carefully and let me know if there are any suggested changes.