stand: Add interp_has_builtin_cmd to see if we have a command

interp_has_builtin_cmd() will try to lookup the passed in command and
returns true if it was found, false otherwise.

Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D36364

stand/common/bootstrap.h

@@ -52,6 +52,7 @@ extern char	command_errbuf[COMMAND_ERRBUFSZ];
 void	interact(void);
 void	interp_emit_prompt(void);
 int	interp_builtin_cmd(int argc, char *argv[]);
+bool	interp_has_builtin_cmd(const char *cmd);
 
 /* Called by interp.c for interp_*.c embedded interpreters */
 int	interp_include(const char *);	/* Execute commands from filename */

stand/common/interp.c

@@ -190,3 +190,12 @@ interp_builtin_cmd(int argc, char *argv[])
 	}
 	return (result);
 }
+
+/*
+ * Return true if the builtin command exists
+ */
+bool
+interp_has_builtin_cmd(const char *cmd)
+{
+	return (interp_lookup_cmd(cmd) != NULL);
+}