ash: better fix for ash -c 'echo 5&' and ash -c 'sleep 5&'

with testcase

shell/ash_test/ash-misc/last_amp.right

@@ -0,0 +1,2 @@
+3
+End

shell/ash_test/ash-misc/last_amp.tests

@@ -0,0 +1,8 @@
+$THIS_SH -c 'echo 3&'
+d=`date`
+while test "`date`" = "$d"; do true; done
+d1=`date`
+$THIS_SH -c 'sleep 1&'
+d2=`date`
+test "$d1" = "$d2" || echo BAD
+echo End