Fix jsonb Unicode escape processing, and in consequence disallow \u0000.

We've been trying to support \u0000 in JSON values since commit
78ed8e03c6, and have introduced increasingly worse hacks to try to
make it work, such as commit 0ad1a81632.  However, it fundamentally
can't work in the way envisioned, because the stored representation looks
the same as for \\u0000 which is not the same thing at all.  It's also
entirely bogus to output \u0000 when de-escaped output is called for.

The right way to do this would be to store an actual 0x00 byte, and then
throw error only if asked to produce de-escaped textual output.  However,
getting to that point seems likely to take considerable work and may well
never be practical in the 9.4.x series.

To preserve our options for better behavior while getting rid of the nasty
side-effects of 0ad1a81632, revert that commit in toto and instead
throw error if \u0000 is used in a context where it needs to be de-escaped.
(These are the same contexts where non-ASCII Unicode escapes throw error
if the database encoding isn't UTF8, so this behavior is by no means
without precedent.)

In passing, make both the \u0000 case and the non-ASCII Unicode case report
ERRCODE_UNTRANSLATABLE_CHARACTER / "unsupported Unicode escape sequence"
rather than claiming there's something wrong with the input syntax.

Back-patch to 9.4, where we have to do something because 0ad1a81632
broke things for many cases having nothing to do with \u0000.  9.3 also has
bogus behavior, but only for that specific escape value, so given the lack
of field complaints it seems better to leave 9.3 alone.

doc/src/sgml/json.sgml

@@ -69,12 +69,14 @@
   regardless of the database encoding, and are checked only for syntactic
   correctness (that is, that four hex digits follow <literal>\u</>).
   However, the input function for <type>jsonb</> is stricter: it disallows
-  Unicode escapes for non-ASCII characters (those
-  above <literal>U+007F</>) unless the database encoding is UTF8.  It also
-  insists that any use of Unicode surrogate pairs to designate characters
-  outside the Unicode Basic Multilingual Plane be correct.  Valid Unicode
-  escapes, except for <literal>\u0000</>, are then converted to the
-  equivalent ASCII or UTF8 character for storage.
+  Unicode escapes for non-ASCII characters (those above <literal>U+007F</>)
+  unless the database encoding is UTF8.  The <type>jsonb</> type also
+  rejects <literal>\u0000</> (because that cannot be represented in
+  <productname>PostgreSQL</productname>'s <type>text</> type), and it insists
+  that any use of Unicode surrogate pairs to designate characters outside
+  the Unicode Basic Multilingual Plane be correct.  Valid Unicode escapes
+  are converted to the equivalent ASCII or UTF8 character for storage;
+  this includes folding surrogate pairs into a single character.
  </para>
 
  <note>
@@ -101,7 +103,7 @@
   constitutes valid <type>jsonb</type> data that do not apply to
   the <type>json</type> type, nor to JSON in the abstract, corresponding
   to limits on what can be represented by the underlying data type.
-  Specifically, <type>jsonb</> will reject numbers that are outside the
+  Notably, <type>jsonb</> will reject numbers that are outside the
   range of the <productname>PostgreSQL</productname> <type>numeric</> data
   type, while <type>json</> will not.  Such implementation-defined
   restrictions are permitted by <acronym>RFC</> 7159.  However, in
@@ -134,7 +136,8 @@
        <row>
         <entry><type>string</></entry>
         <entry><type>text</></entry>
-        <entry>See notes above concerning encoding restrictions</entry>
+        <entry><literal>\u0000</> is disallowed, as are non-ASCII Unicode
+         escapes if database encoding is not UTF8</entry>
        </row>
        <row>
         <entry><type>number</></entry>