Add CSV table output mode in psql.

"\pset format csv", or --csv, selects comma-separated values table format.
This is compliant with RFC 4180, except that we aren't too picky about
whether the record separator is LF or CRLF; also, the user may choose a
field separator other than comma.

This output format is directly compatible with the server's COPY CSV
format, and will also be useful as input to other programs.  It's
considerably safer for that purpose than the old recommendation to
use "unaligned" format, since the latter couldn't handle data
containing the field separator character.

Daniel Vérité, reviewed by Fabien Coelho and David Fetter, some
tweaking by me

Discussion: https://postgr.es/m/a8de371e-006f-4f92-ab72-2bbe3ee78f03@manitou-mail.org

src/include/fe_utils/print.h

@@ -28,6 +28,7 @@ enum printFormat
 	PRINT_NOTHING = 0,			/* to make sure someone initializes this */
 	PRINT_ALIGNED,
 	PRINT_ASCIIDOC,
+	PRINT_CSV,
 	PRINT_HTML,
 	PRINT_LATEX,
 	PRINT_LATEX_LONGTABLE,
@@ -112,6 +113,7 @@ typedef struct printTableOpt
 	const printTextFormat *line_style;	/* line style (NULL for default) */
 	struct separator fieldSep;	/* field separator for unaligned text mode */
 	struct separator recordSep; /* record separator for unaligned text mode */
+	char		csvFieldSep[2]; /* field separator for csv format */
 	bool		numericLocale;	/* locale-aware numeric units separator and
 								 * decimal marker */
 	char	   *tableAttr;		/* attributes for HTML <table ...> */