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.

plan for eviction.