Store IF depth relative to each fstack context

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

test/asm/break.asm

@@ -20,3 +20,4 @@ rept 1
     break
   no endc
 endr
+println "done"

test/asm/break.err

@@ -2,4 +2,5 @@ warning: break.asm(9): [-Wuser]
     done 5
 warning: break.asm(17): [-Wuser]
     OK
-error: Unterminated IF construct (1 levels)!
+FATAL: break.asm(18) -> break.asm::REPT~1(23):
+    Ended block with 1 unterminated IF construct

test/asm/unterminated-if-include.inc

@@ -0,0 +1,3 @@
+if 1
+  println "inside include"
+; no endc

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"

test/asm/unterminated-if.err

@@ -0,0 +1,2 @@
+FATAL: unterminated-if.asm(13) -> unterminated-if-include.inc(4):
+    Ended block with 1 unterminated IF construct

test/asm/unterminated-if.out

@@ -0,0 +1,2 @@
+B
+inside include