pgsql: Add new escaping functions PQescapeLiteral and

Log Message:
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Add new escaping functions PQescapeLiteral and PQescapeIdentifier.

PQescapeLiteral is similar to PQescapeStringConn, but it relieves the
caller of the need to know how large the output buffer should be, and
it provides the appropriate quoting (in addition to escaping special
characers within the string).  PQescapeIdentifier provides similar
functionality for escaping identifiers.

Per recent discussion with Tom Lane.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        libpq.sgml (r1.294 -> r1.295)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.294&r2=1.295)
    pgsql/src/interfaces/libpq:
        exports.txt (r1.23 -> r1.24)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/exports.txt?r1=1.23&r2=1.24)
        fe-exec.c (r1.206 -> r1.207)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-exec.c?r1=1.206&r2=1.207)
        libpq-fe.h (r1.148 -> r1.149)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-fe.h?r1=1.148&r2=1.149)