don't fail posix_spawn on failed close

the resolution of austin group issue #370 removes the requirement that
posix_spawn fail when the close file action is performed on an
already-closed fd. since there are no other meaningful errors for
close, just ignoring the return value completely is the simplest fix.

src/process/posix_spawn.c

@@ -102,8 +102,7 @@ static int child(void *args_vp)
 			}
 			switch(op->cmd) {
 			case FDOP_CLOSE:
-				if ((ret=__syscall(SYS_close, op->fd)))
-					goto fail;
+				__syscall(SYS_close, op->fd);
 				break;
 			case FDOP_DUP2:
 				if ((ret=__sys_dup2(op->srcfd, op->fd))<0)