Add header matching mode to COPY FROM

COPY FROM supports the HEADER option to silently discard the header
line from a CSV or text file.  It is possible to load by mistake a
file that matches the expected format, for example, if two text
columns have been swapped, resulting in garbage in the database.

This adds a new option value HEADER MATCH that checks the column names
in the header line against the actual column names and errors out if
they do not match.

Author: Rémi Lapeyre <remi.lapeyre@lenstra.fr>
Reviewed-by: Daniel Verite <daniel@manitou-mail.org>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/CAF1-J-0PtCWMeLtswwGV2M70U26n4g33gpe1rcKQqe6wVQDrFA@mail.gmail.com

src/include/commands/copy.h

@@ -19,6 +19,17 @@
 #include "parser/parse_node.h"
 #include "tcop/dest.h"
 
+/*
+ * Represents whether a header line should be present, and whether it must
+ * match the actual names (which implies "true").
+ */
+typedef enum CopyHeaderChoice
+{
+	COPY_HEADER_FALSE = 0,
+	COPY_HEADER_TRUE,
+	COPY_HEADER_MATCH,
+} CopyHeaderChoice;
+
 /*
  * A struct to hold COPY options, in a parsed form. All of these are related
  * to formatting, except for 'freeze', which doesn't really belong here, but
@@ -32,7 +43,7 @@ typedef struct CopyFormatOptions
 	bool		binary;			/* binary format? */
 	bool		freeze;			/* freeze rows on loading? */
 	bool		csv_mode;		/* Comma Separated Value format? */
-	bool		header_line;	/* header line? */
+	CopyHeaderChoice header_line;	/* header line? */
 	char	   *null_print;		/* NULL marker string (server encoding!) */
 	int			null_print_len; /* length of same */
 	char	   *null_print_client;	/* same converted to file encoding */