when a zero PDE is encountered while searching for present PTEs to free,
resume searching at first entry of the next page table instead of the
current entry of the next page table.

On platforms with NaCl support, ld.gold -V will return a "elf_i386_nacl" in
addition to "elf_i386", which will make the build fail.

fix bugs where caller incorrectly released lock on error return path.

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)