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/test/regress/sql/psql.sql

@@ -501,6 +501,54 @@ execute q;
 
 deallocate q;
 
+-- test csv output format
+
+\pset format csv
+
+\pset border 1
+\pset expanded off
+\d psql_serial_tab_id_seq
+\pset tuples_only true
+\df exp
+\pset tuples_only false
+\pset expanded on
+\d psql_serial_tab_id_seq
+\pset tuples_only true
+\df exp
+\pset tuples_only false
+
+prepare q as
+  select 'some"text' as "a""title", E'  <foo>\n<bar>' as "junk",
+         '   ' as "empty", n as int
+  from generate_series(1,2) as n;
+
+\pset expanded off
+execute q;
+
+\pset expanded on
+execute q;
+
+deallocate q;
+
+-- special cases
+\pset expanded off
+select 'comma,comma' as comma, 'semi;semi' as semi;
+\pset csv_fieldsep ';'
+select 'comma,comma' as comma, 'semi;semi' as semi;
+select '\.' as data;
+\pset csv_fieldsep '.'
+select '\' as d1, '' as d2;
+
+-- illegal csv separators
+\pset csv_fieldsep ''
+\pset csv_fieldsep '\0'
+\pset csv_fieldsep '\n'
+\pset csv_fieldsep '\r'
+\pset csv_fieldsep '"'
+\pset csv_fieldsep ',,'
+
+\pset csv_fieldsep ','
+
 -- test html output format
 
 \pset format html