Prep .c files for removing the #includes from */archdep.h

Prep .c files for removing the #includes from */archdep.h
 * replace #include "archdep.h" with #includes of what is used, pulling in
   "syscall.h", "util.h", and "archdep.h" as needed
 * delete #include <sys/syscall.h> from syscall.h
 * only pull in <sys/stat.h> to the three files that use _dl_fstat(),
   forward declare struct stat in syscall.h for the others
 * NBBY is for <sys/select.h> macros; just use '8' in dl_printf.c
 * <machine/vmparam.h> is only needed on i386; conditionalize it
 * stop using __LDPGSZ: use _MAX_PAGE_SHIFT (already used by malloc.c)
   where necessary
 * delete other bogus #includes, order legit per style: <sys/*> then
   <*/*>, then <*>, then "*"

dir.c improvement from jsg@
ok and testing assistance deraadt@