pgsql: The following patch should allow UPDATE_INTERVAL to be specified

Log Message:
-----------
The following patch should allow UPDATE_INTERVAL to be specified on the
command line. We find this useful because we frequently deal with
thousands of tables in an environment where neither the databases nor
the tables are updated frequently. This helps allow us to cut down on
the overhead of updating the list for every other primary loop of
pg_autovacuum.

I chose -i as the command-line argument and documented it briefly in
the README.

The patch was applied to the 7.4.7 version of pg_autovacuum in contrib.

Thomas F.O'Connell

Modified Files:
--------------
pgsql/contrib/pg_autovacuum:
README.pg_autovacuum (r1.13 -> r1.14)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_autovacuum/README.pg_autovacuum.diff?r1=1.13&r2=1.14)
pg_autovacuum.c (r1.30 -> r1.31)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_autovacuum/pg_autovacuum.c.diff?r1=1.30&r2=1.31)
pg_autovacuum.h (r1.14 -> r1.15)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_autovacuum/pg_autovacuum.h.diff?r1=1.14&r2=1.15)