further libexec generalization

. new mode for sys_memset: include process so memset can be
	  done in physical or virtual address space.
	. add a mode to mmap() that lets a process allocate uninitialized
	  memory.
	. this allows an exec()er (RS, VFS, etc.) to request uninitialized
	  memory from VM and selectively clear the ranges that don't come
	  from a file, leaving no uninitialized memory left for the process
	  to see.
	. use callbacks for clearing the process, clearing memory in the
	  process, and copying into the process; so that the libexec code
	  can be used from rs, vfs, and in the future, kernel (to load vm)
	  and vm (to load boot-time processes)

include/sys/mman.h

@@ -59,6 +59,7 @@ typedef	__off_t		off_t;		/* file offset */
 
 #define MAP_FIXED      0x0200  /* require mapping to happen at hint */
 #define MAP_THIRDPARTY	0x0400		/* perform on behalf of any process */
+#define MAP_UNINITIALIZED 0x0800	/* do not clear memory */
 
 /*
  * Error indicator returned by mmap(2)