Change ALPN protocol ID to IANA-approved "postgresql"

"TBD-pgsql" was a placeholder until the IANA registration was
approved.

Discussion: https://www.postgresql.org/message-id/87jzk2hj2n.fsf%40wibble.ilmari.org
Discussion: https://mailarchive.ietf.org/arch/msg/tls-reg-review/9LWPzQfOpbc8dTT7vc9ahNeNaiw/

doc/src/sgml/libpq.sgml

@@ -2949,7 +2949,7 @@ const char *PQsslAttribute(const PGconn *conn, const char *attribute_name);
            <para>
             Application protocol selected by the TLS Application-Layer
             Protocol Negotiation (ALPN) extension.  The only protocol
-            supported by libpq is <literal>TBD-pgsql</literal>, so this is
+            supported by libpq is <literal>postgresql</literal>, so this is
             mainly useful for checking whether the server supported ALPN or
             not. Empty string if ALPN was not used.
            </para>

doc/src/sgml/protocol.sgml

@@ -1564,7 +1564,7 @@ SELCT 1/0;<!-- this typo is intentional -->
      required to use the ALPN extension defined
      by <ulink url="https://tools.ietf.org/html/rfc7301">RFC 7301</ulink> to
      protect against protocol confusion attacks.
-     The <productname>PostgreSQL</productname> protocol is "TBD-pgsql" as
+     The <productname>PostgreSQL</productname> protocol is "postgresql" as
      registered
      at <ulink url="https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml#alpn-protocol-ids">IANA
      TLS ALPN Protocol IDs</ulink> registry.

src/include/libpq/pqcomm.h

@@ -156,8 +156,8 @@ typedef struct CancelRequestPacket
  *
  * The #define can be used to initialize a char[] vector to use directly in the API
  */
-#define PG_ALPN_PROTOCOL "TBD-pgsql"
-#define PG_ALPN_PROTOCOL_VECTOR { 9, 'T','B','D','-','p','g','s','q','l' }
+#define PG_ALPN_PROTOCOL "postgresql"
+#define PG_ALPN_PROTOCOL_VECTOR { 10, 'p','o','s','t','g','r','e','s','q','l' }
 
 /*
  * A client can also start by sending a SSL or GSSAPI negotiation request to