Rename reserved param aom_codec_enc_config_default

Rename the 'reserved' parameter of aom_codec_enc_config_default() to
'usage'.

Declare all the usage parameters and struct members to have the unsigned
int type for consistency. This eliminates the int cast and the > INT_MAX
check in aom_codec_enc_config_default().

Remove the internal type aom_codec_enc_cfg_map_t because its usage field
is always set equal to cfg.g_usage. Use aom_codec_enc_cfg_t directly.

Change-Id: I65326cfd4851e0980f97e2c1de7699d34c7c5364
diff --git a/apps/aomenc.h b/apps/aomenc.h
index ba7bf0a..a38258b 100644
--- a/apps/aomenc.h
+++ b/apps/aomenc.h
@@ -38,7 +38,7 @@
   const struct AvxInterface *codec;
   int passes;
   int pass;
-  int usage;
+  unsigned int usage;
   ColorInputType color_type;
   int quiet;
   int verbose;