This code is inefficient, it doesn't guarantee to find an unmapped
area if one exists, and it only works for crange.

in terms of performance on real hardware..  need to think more.

to avoid lost tlbflush IPIs.

..allocfd might still cause bottlenecks as threads race to allocate
FD slots..

referenced includes a "tryinc" member function that returns false if
the reference count is 0, otherwise it increments the ref. count and
returns true.  As a bonus, the implementation does not use cmpxchg
loops/retries.

Conflicts:
	kernel/sysfile.cc

O_WAIT -- wait for the file to exist
O_WAIT -- wake up anyone waiting for the create
nsh -- same as sh.c but opens with O_WAIT

This results in fewer garbaled userspace console outputs.