Fixed alignment of user section in linekr script

.user should be padded to a page boundary for easy copying.
abstracted common.h
preliminary work on kprintf
More thread arch abstratcion work
Makefile arch hack, defines ARCH_I386
Added note about arch specific stdint
Removed i386 specific stuff
Major work on thread/exec.c (sys_execve)

Changed how arguments are passed to program. cleaned up duplicate
code and removed some FIXMEs, though others remain. New executable
gets a new address space now, but we don't clean up the old one
yet.