Fix handling of '+' by 'cygcheck -p'

The form data sent to the server should be application/x-www-form-urlencoded

This replaces spaces with '+' before being RFC 1738 encoded, so a literal
'+' must be %-encoded also.

See https://cygwin.com/ml/cygwin/2014-01/msg00287.html et seq.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>

winsup/utils/cygcheck.cc

@@ -2009,8 +2009,8 @@ check_keys ()
   return 0;
 }
 
-/* RFC1738 says that these do not need to be escaped.  */
-static const char safe_chars[] = "$-_.+!*'(),";
+/* These do not need to be escaped in application/x-www-form-urlencoded */
+static const char safe_chars[] = "$-_.!*'(),";
 
 /* the URL to query.  */
 static const char base_url[] =