arguments passed to printf are more generic

On systems that /bin/sh is served by shells other than bash, or 
shells that don't implement the same syntax as the bash printf builtin,
printf -- fails to produce the output necessary for standalone operation.

The test case for this was SmartOS, which uses ksh93 as its /bin/sh.

This change uses the more generic method of passing a format parameter
of a single string, and then the argument to replace it with.

acme.sh

@@ -2357,7 +2357,7 @@ _startserver() {
 echo 'HTTP/1.0 200 OK'; \
 echo 'Content-Length\: $_content_len'; \
 echo ''; \
-printf -- '$content';" &
+printf '%s' '$content';" &
   serverproc="$!"
 }