pgsql: Allow the types of parameters to PREPARE to be inferred.

Log Message:
-----------
Allow the types of parameters to PREPARE to be inferred. If a parameter's
data type is unspecified or is declared to be "unknown", the type will
be inferred from the context in which the parameter is used. This was
already possible for protocol-level prepared statements.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        deallocate.sgml (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/deallocate.sgml.diff?r1=1.7&r2=1.8)
        execute.sgml (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/execute.sgml.diff?r1=1.12&r2=1.13)
        prepare.sgml (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/prepare.sgml.diff?r1=1.17&r2=1.18)
    pgsql/src/backend/parser:
        analyze.c (r1.327 -> r1.328)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/analyze.c.diff?r1=1.327&r2=1.328)
    pgsql/src/test/regress/expected:
        prepare.out (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/prepare.out.diff?r1=1.6&r2=1.7)
    pgsql/src/test/regress/sql:
        prepare.sql (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/prepare.sql.diff?r1=1.3&r2=1.4)