27129: fix status at start of function, command subst, etc.

Test/A01grammar.ztst

@@ -553,3 +553,9 @@
 0:Last status of successfully executed "." file is retained
 >1
 >0
+
+  echo 'echo $?' >dot_status
+  false
+  . ./dot_status
+0:"." file sees status from previous command
+>1

Test/C04funcdef.ztst

@@ -15,6 +15,13 @@
   fnz
 0:Empty function body resets status
 
+  fn3() { return 3; }
+  fnstat() { print $?; }
+  fn3
+  fnstat
+0:Status is not reset on non-empty function body
+>3
+
   function f$$ () {
     print regress expansion of function names
   }

Test/D08cmdsubst.ztst

@@ -93,3 +93,8 @@
  false
  ``
 0:Empty command substitution resets status
+
+ false
+ echo `echo $?`
+0:Non-empty command substitution inherits status
+>1