Switch KI protection default from "unprotected" to "protected"

This is mostly motivated by wanting to allow guest-mode (gh-1551) to
use our KI handling, where the host loop will presumably not be using
any of our KI protection markers. But I figured I'd split it out into
a separate PR rather than mix changes to the tricky KI code together
with changes to the tricky core run loop.

No newsfragment because I don't think this change is observable by
users.

docs/source/reference-lowlevel.rst

@@ -295,11 +295,15 @@ delivered).
 So that's great, but – how do we know whether we're in one of the
 sensitive parts of the program or not?
 
-This is determined on a function-by-function basis. By default, a
-function is protected if its caller is, and not if its caller isn't;
-this is helpful because it means you only need to override the
-defaults at places where you transition from protected code to
-unprotected code or vice-versa.
+This is determined on a function-by-function basis. By default:
+
+- The top-level function in regular user tasks is unprotected.
+- The top-level function in system tasks is protected.
+- If a function doesn't specify otherwise, then it inherits the
+  protection state of its caller.
+
+This means you only need to override the defaults at places where you
+transition from protected code to unprotected code or vice-versa.
 
 These transitions are accomplished using two function decorators:
 

trio/_core/_ki.py

@@ -90,7 +90,7 @@ def ki_protection_enabled(frame):
         if frame.f_code.co_name == "__del__":
             return True
         frame = frame.f_back
-    return False
+    return True
 
 
 def currently_ki_protected():

trio/_core/_run.py

@@ -1678,7 +1678,6 @@ def run(
         system_context=system_context,
     )
     GLOBAL_RUN_CONTEXT.runner = runner
-    locals()[LOCALS_KEY_KI_PROTECTION_ENABLED] = True
 
     # KI handling goes outside the core try/except/finally to avoid a window
     # where KeyboardInterrupt would be allowed and converted into an

trio/_core/tests/test_ki.py

@@ -222,8 +222,8 @@ async def agen_unprotected3():
 
 
 # Test the case where there's no magic local anywhere in the call stack
-def test_ki_enabled_out_of_context():
-    assert not _core.currently_ki_protected()
+def test_ki_disabled_out_of_context():
+    assert _core.currently_ki_protected()
 
 
 def test_ki_disabled_in_del():
@@ -234,8 +234,15 @@ def __del__():
         assert _core.currently_ki_protected()
         assert nestedfunction()
 
+    @_core.disable_ki_protection
+    def outerfunction():
+        assert not _core.currently_ki_protected()
+        assert not nestedfunction()
+        __del__()
+
     __del__()
-    assert not nestedfunction()
+    outerfunction()
+    assert nestedfunction()
 
 
 def test_ki_protection_works():