Stringify all the CONF_foo identifiers, for debugging.

When dumping out the contents of a Conf, it's useful not to have to
guess what the integer indices mean.

By putting these identifiers in a separate array in its own library
module, I should avoid them getting linked in to production binaries
to take up space, as long as conf_id() is only called from inside
debug() statements. And to enforce _that_, it isn't even declared in a
header file unless you #define DEBUG.

utils/CMakeLists.txt

@@ -16,6 +16,7 @@ add_sources_from_current_dir(utils
   conf.c
   conf_data.c
   conf_dest.c
+  conf_debug.c
   conf_launchable.c
   ctrlparse.c
   ctrlset_normalise.c

utils/conf_debug.c

@@ -0,0 +1,15 @@
+#include "putty.h"
+
+#define CONF_OPTION(id, ...) "CONF_" #id,
+
+static const char *const conf_debug_identifiers[] = {
+    #include "conf.h"
+};
+
+const char *conf_id(int key)
+{
+    size_t i = key;
+    if (i < lenof(conf_debug_identifiers))
+        return conf_debug_identifiers[i];
+    return "CONF_!outofrange!";
+}