pgsql: Increase the maximum value of extra_float_digits to 3, and have

Log Message:
-----------
Increase the maximum value of extra_float_digits to 3, and have pg_dump
use that value when the backend is new enough to allow it. This responds
to bug report from Keh-Cheng Chu pointing out that although 2 extra digits
should be sufficient to dump and restore float8 exactly, it is possible to
need 3 extra digits for float4 values.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.226 -> r1.227)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.226&r2=1.227)
pgsql/src/backend/utils/adt:
float.c (r1.162 -> r1.163)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/float.c?r1=1.162&r2=1.163)
geo_ops.c (r1.104 -> r1.105)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/geo_ops.c?r1=1.104&r2=1.105)
pgsql/src/backend/utils/misc:
guc.c (r1.516 -> r1.517)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.516&r2=1.517)
postgresql.conf.sample (r1.265 -> r1.266)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.265&r2=1.266)
pgsql/src/bin/pg_dump:
pg_dump.c (r1.546 -> r1.547)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c?r1=1.546&r2=1.547)