pgsql: Surrogate pair support for U& string and identifier syntax This

Log Message:
-----------
Surrogate pair support for U& string and identifier syntax

This is mainly to make the functionality consistent with the proposed \u
escape syntax.

Modified Files:
--------------
pgsql/doc/src/sgml:
syntax.sgml (r1.134 -> r1.135)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/syntax.sgml?r1=1.134&r2=1.135)
pgsql/src/backend/parser:
scan.l (r1.157 -> r1.158)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/scan.l?r1=1.157&r2=1.158)