Fix pg_restore's direct-to-database mode for standard_conforming_strings.

Fix pg_restore's direct-to-database mode for standard_conforming_strings.

pg_backup_db.c contained a mini SQL lexer with which it tried to identify
boundaries between SQL commands, but that code was not designed to cope
with standard_conforming_strings, and would get the wrong answer if a
backslash immediately precedes a closing single quote in such a string,
as per report from Julian Mehnle.  The bug only affects direct-to-database
restores from archive files made with standard_conforming_strings = on.

Rather than complicating the code some more to try to fix that, let's just
rip it all out.  The only reason it was needed was to cope with COPY data
embedded into ordinary archive entries, which was a layout that was used
only for about the first three weeks of the archive format's existence,
and never in any production release of pg_dump.  Instead, just rely on the
archive file layout to tell us whether we're printing COPY data or not.

This bug represents a data corruption hazard in all releases in which
standard_conforming_strings can be turned on, ie 8.2 and later, so
back-patch to all supported branches.