as a direct index into cpus.  Record apicid in struct cpu and have cpunum() look
for it. Replace cpu->id with cpunum() everywhere, and replace cpu->id with cpu->apicid.
Thanks to Xi Wang.

add to test that they indeed trap in user space
thanks to pablogventura@gmail.com and nwolovick@gmail.com

 for f in *.{h,c}; do sed -i .sed 's/[[:blank:]]*$//' $f; done
(Thanks to Nicolás Wolovick)

that is a multiple of the sector size.

however, processes still modify their own p->* without locking

* use xchg only for its atomicness.
* use __sync_synchronize() for both CPU and compiler barrier.

fix Regehr complaint

Use cons.lock for everything.  This eliminates the possibility that two CPUS
independently, simultaneously manipulate the CRTC in cgaputc.

Fix comments