Remove the stack guard page.  Processes are now contiguous from 0 to proc->sz, which means our syscall argument validation is correct.  Add a pointer validation test and remove the stack test, which tested for the guard page.

increase PHYSTOP

put an invalid page below the stack
have fork() handle invalid pages

(maybe this never worked, but it works now)

group locks into structs they protect.
few naming nits.

the macro expansion of "char *cp;" turned into
char *(curproc[cpu()]);  which declares a dynamically
sized array of char* called curproc.

so then &cp == &(curproc[cpu()]) was actually a
stack variable as "expected".  it was one past the
end of the array, but the implicit alloca allocated
more than was necessary.

do not tell me that making cp a #define was a bad idea.
there are worse problems to fix.  more on that later.

test files with holes (not ok)

fix bug in test code of malloc

test malloc

cd in shell
nits in mkdir, ls, etc.

each cpu[] has its own gdt and tss
no per-proc gdt or tss, re-write cpu's in scheduler (you win, cliff)
main0() switches to cpu[0].mpstack

don't hold fd table lock across idecref() (latter does block i/o)
idecref calls iput() in case last ref -> freeing inode
dir size is 512 * # blocks, so readi/writei &c work
unlink deletes dirent even if ip->nlink > 0

usertests for concurrent create/delete, and read() after unlink()

no more cons_put system calls
usertests tests two processes writing files