layers: adjust VU database/scripts for impl flag

Since the VU database audit, "U" is no longer valid as a value for
the check_implemented status; only "Y" or "N" are valid.

Adjust the spec.py generator script to default to "N" for newly added
Valid Usages.

Adjust database stats script to emit an error message if the
check_implemented value is anything other than "Y" or "N".
Added some whitespace between functional areas.

Update the database to change recently VU lines from "U" to "N".

Change-Id: I72929362d67ddea862cb380e1bfcd0b475531d56
diff --git a/layers/spec.py b/layers/spec.py
index 0ee4e56..51d2b4c 100644
--- a/layers/spec.py
+++ b/layers/spec.py
@@ -238,14 +238,14 @@
         db_lines.append("# The format of the lines is:")
         db_lines.append("# <error_enum>%s<check_implemented>%s<testname>%s<api>%s<errormsg>%s<note>" % (self.delimiter, self.delimiter, self.delimiter, self.delimiter, self.delimiter))
         db_lines.append("# error_enum: Unique error enum for this check of format %s<uniqueid>" % validation_error_enum_name)
-        db_lines.append("# check_implemented: 'Y' if check has been implemented in layers, 'U' for unknown, or 'N' for not implemented")
+        db_lines.append("# check_implemented: 'Y' if check has been implemented in layers, or 'N' for not implemented")
         db_lines.append("# testname: Name of validation test for this check, 'Unknown' for unknown, or 'None' if not implmented")
         db_lines.append("# api: Vulkan API function that this check is related to")
         db_lines.append("# errormsg: The unique error message for this check that includes spec language and link")
         db_lines.append("# note: Free txt field with any custom notes related to the check in question")
         for enum in sorted(self.val_error_dict):
-            # Default to unknown if check or test are implemented, then update below if appropriate
-            implemented = 'U'
+            # Default check/test implementation status to N/Unknown, then update below if appropriate
+            implemented = 'N'
             testname = 'Unknown'
             note = ''
             implicit = self.val_error_dict[enum]['implicit']