Fractions in GUC variables

This patch implements the first wiki/Todo Configuration Files item
"Consider normalizing fractions in postgresql.conf, perhaps using '%'".

The "Fractions in GUC variables" discussion is here.

http://www.postgresql.org/message-id/467132CF.9020501@enterprisedb.com

This patch implements expressing GUC variables as percents in
postgresql.conf.

autovacuum_vacuum_scale_factor = 20% # percent of table size before vacuum
autovacuum_analyze_scale_factor = 10% # percent of table size before
analyze

As you can see the postgresql.conf file and the documentation read more
naturally. I added a regression test to guc.sql. The sql interface also
accepts both numeric and percent forms although the percent form must be
quoted because '%' is an operator.

show cursor_tuple_fraction; --> 10%
set cursor_tuple_fraction = .15; --> 15%
set cursor_tuple_fraction = '33%'; --> 33%

I tagged four configuration variables to display as percents.

The attached patch applies cleanly against master and passes all regression
tests including two new tests in guc.sql.