Add recursion limit for INCLUDE and macros

(And REPT.)
Not exactly a *recursion* limit, more like a *stack depth* limit,
but calling it "recursion" conveys its purpose better.
The default of 64 is super overkill: even in a a project with
what I believe to be above-average levels of nesting, the
level only peaked at 6.
Keeping in mind the purpose of this is to catch infinite
recursion, which is still caught quickly (in usual cases, anyways),
this default seems sensible.
And it passes tests. What more do you need?

include/asm/fstack.h

@@ -35,6 +35,8 @@ struct sContext {
 	uint32_t nREPTBlockSize;
 };
 
+extern unsigned int nMaxFileStackDepth;
+
 void fstk_RunInclude(char *tzFileName);
 void fstk_RunMacroArg(int32_t s);
 void fstk_Init(char *s);