Base status -b off the parser execution stack

Instead of globally marking the state as "in block" when evaluating
blocks/functions, update the "in block" status when pushing/popping
blocks on the parser stack.

Fixes #1729.

On a side note, `status -b` is actually pretty useless, because it
always returns 0 inside of a function (even without this patch).

tests/status.in

@@ -1,9 +1,17 @@
 # vim: set filetype=fish:
 
+status -b
+or echo 'top level'
+
+begin
+    status -b
+end
+and echo 'block'
+
 # Issue #1728
 # Bad file redirection on a block causes `status --is-block` to return 0 forever.
 begin; end >/ # / is a directory, it can't be opened for writing
 status -b
-and echo 'block'
+and echo 'unexpected block'
 
 true

tests/status.out

@@ -0,0 +1,2 @@
+top level
+block