pgsql: Perform post-escaping encoding validity checks on SQL literals

Log Message:
-----------
Perform post-escaping encoding validity checks on SQL literals and COPY input
so that invalidly encoded data cannot enter the database by these means.

Modified Files:
--------------
    pgsql/src/backend/commands:
        copy.c (r1.286 -> r1.287)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/copy.c?r1=1.286&r2=1.287)
    pgsql/src/backend/parser:
        scan.l (r1.140 -> r1.141)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/scan.l?r1=1.140&r2=1.141)