Store IF depth relative to each fstack context

This disallows starting/ending an IF inside an
INCLUDEd file or a macro expansion

test/asm/unterminated-if.asm

@@ -0,0 +1,15 @@
+if 1
+  if 0
+    println "A"
+  elif 1
+    println "B"
+  else
+    println "C"
+  endc
+else
+  println "D"
+endc
+
+INCLUDE "unterminated-if-include.inc"
+
+println "done"