add better support for IWYU and fix things

Remove the "make iwyu" build target. Move the functionality into the
recently introduced lint.fish script. Fix a lot, but not all, of the
include-what-you-use errors. Specifically, it fixes all of the IWYU errors
on my OS X server but only removes some of them on my Ubuntu 14.04 server.

Fixes #2957

src/parser_keywords.h

@@ -2,10 +2,11 @@
 
 Functions having to do with parser keywords, like testing if a function is a block command.
 */
-
 #ifndef FISH_PARSER_KEYWORD_H
 #define FISH_PARSER_KEYWORD_H
 
+#include <stdbool.h>
+
 #include "common.h"
 
 /**