echo: fix ENOSPC detection and some iffy code in \NNN handling

function                                             old     new   delta
echo_main                                            330     302     -28

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/testsuite/echo/echo-prints-slash_041 b/testsuite/echo/echo-prints-slash_041
new file mode 100644
index 0000000..1d70cec
--- /dev/null
+++ b/testsuite/echo/echo-prints-slash_041
@@ -0,0 +1,3 @@
+# FEATURE: CONFIG_FEATURE_FANCY_ECHO
+
+test "`busybox echo -ne '\041z' | od -t x1 | head -n 1`" = "0000000 21 7a"