psql: Improve \pset without arguments

Revert the output of the individual backslash commands that change print
settings back to the 9.3 way (not showing the command name in
parentheses).  Implement \pset without arguments separately, showing all
settings with values in a table form.

src/test/regress/expected/psql.out

@@ -54,23 +54,25 @@ no rows returned for \gset
 \unset FETCH_COUNT
 -- show all pset options
 \pset
-Border style (border) is 1.
-Target width (columns) unset.
-Expanded display (expanded) is off.
-Field separator (fieldsep) is "|".
-Default footer (footer) is on.
-Output format (format) is aligned.
-Line style (linestyle) is ascii.
-Null display (null) is "".
-Locale-adjusted numeric output (numericlocale) is off.
-Pager (pager) is used for long output.
-Record separator (recordsep) is <newline>.
-Table attributes (tableattr) unset.
-Title (title) unset.
-Tuples only (tuples_only) is off.
-Unicode border linestyle is "single".
-Unicode column linestyle is "single".
-Unicode border linestyle is "single".
+border                   1
+columns                  0
+expanded                 off
+fieldsep                 '|'
+fieldsep_zero            off
+footer                   on
+format                   aligned
+linestyle                ascii
+null                     ''
+numericlocale            off
+pager                    1
+recordsep                '\n'
+recordsep_zero           off
+tableattr                
+title                    
+tuples_only              off
+unicode_border_linestyle single
+unicode_column_linestyle single
+unicode_header_linestyle single
 -- test multi-line headers, wrapping, and newline indicators
 prepare q as select array_to_string(array_agg(repeat('x',2*n)),E'\n') as "ab