Add E'' syntax so eventually normal strings can treat backslashes

literally.

Add GUC variables:

        "escape_string_warning" - warn about backslashes in non-E strings
        "escape_string_syntax" - supports E'' syntax?
        "standard_compliant_strings" - treats backslashes literally in ''

Update code to use E'' when escapes are used.

src/backend/utils/misc/postgresql.conf.sample

@@ -330,6 +330,7 @@
 #regex_flavor = advanced	# advanced, extended, or basic
 #sql_inheritance = true
 #default_with_oids = false
+#escape_string_warning = false
 
 # - Other Platforms & Clients -