libbb: add a function to make a copy of a region of memory

Introduce a library routine to package the idiom:

    p = xmalloc(b, n);
    memcpy(p, b, n);

and use it where possible.  The example in traceroute used xzalloc
but it didn't need to.

function                                             old     new   delta
xmemdup                                                -      32     +32
last_main                                            834     826      -8
make_device                                         2321    2311     -10
common_traceroute_main                              3698    3685     -13
readtoken1                                          3182    3168     -14
procps_scan                                         1222    1206     -16
forkchild                                            655     638     -17
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/6 up/down: 32/-78)            Total: -46 bytes

Signed-off-by: Ron Yorston <rmy@frippery.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash.c

@@ -4751,7 +4751,7 @@ forkchild(struct job *jp, union node *n, int mode)
 		 * Our solution: ONLY bare $(trap) or `trap` is special.
 		 */
 		/* Save trap handler strings for trap builtin to print */
-		trap_ptr = memcpy(xmalloc(sizeof(trap)), trap, sizeof(trap));
+		trap_ptr = xmemdup(trap, sizeof(trap));
 		/* Fall through into clearing traps */
 	}
 	clear_traps();