I have implemented the suggestion Jay made a couple of weeks ago
about allowing partial allocation in assign_wrapper, and retrying with a
modified set of "fixed" nodes.

My basic approach was to change nalloc to optionally allow partial
allocations, returning the number of nodes that could not be allocated as
its return value. In assign_wrapper, I determine which nodes we were able
to get (in each loop), set their allocstate to INIT_DIRTY, augment the
fixed_node set, and recreate the top file. Then I try again, up to the
current number of maxtries. If assign fails with an unretryable error, or
if we could not nalloc a user directed fixed node, then I stop right away
since the experiment is not going to map (in the near term) if the fixed
node list cannot be allocated.

I am confident that this works okay, although testing is a little
difficult. The main problem is how this interacts with experiment modify.
Chad's implementation is that a modify can be reverted (recovered from)
only as long as the DB is not modified by assign_wrapper. Well, a partial
allocation, followed by failure, obviously modifies the DB, and so is
deemed not recoverable. I am still trying to figure out the effects of
this, and whether I can relax this requirement, but in the meantime
lets install it and see what happens (won't affect many people).