nsh/nshlib: Add alias support for nsh

This adds support for string aliases into nsh. There are some nuances that
are not handled correctly yet:

- Reserved words can be overloaded, which is a clear POSIX violation

nshlib/nsh_console.h

@@ -33,6 +33,8 @@
 #include <stdbool.h>
 #include <errno.h>
 
+#include <nuttx/queue.h>
+
 /****************************************************************************
  * Pre-processor Definitions
  ****************************************************************************/
@@ -119,6 +121,14 @@ struct nsh_vtbl_s
   char iobuffer[IOBUFFERSIZE];
 #endif
 
+#ifdef CONFIG_NSH_ALIAS
+  /* Shell alias support */
+
+  struct nsh_alias_s atab[CONFIG_NSH_ALIAS_MAX_AMOUNT];
+  struct sq_queue_s  alist;
+  struct sq_queue_s  afreelist;
+#endif
+
   /* Parser state data */
 
   struct nsh_parser_s np;