Improved automation of INSTALL file generation.

There's now only one transition value and transition function.
NULL handling in aggregates is a lot cleaner.  Also, use Numeric
accumulators instead of integer accumulators for sum/avg on integer
datatypes --- this avoids overflow at the cost of being a little slower.
Implement VARIANCE() and STDDEV() aggregates in the standard backend.

Also, enable new LIKE selectivity estimators by default.  Unrelated
change, but as long as I had to force initdb anyway...

The .DEFAULT rule in backend/Makefile is harmful -- removed.
Replace `::' rules by `:'.

Fix references to pg_hba.conf(5) since that is no longer a man page.
Make minor repairs to "Compatibility" subsections.

Add information on SET SESSION CHARACTERISTICS.
Fix references to pg_hba.conf(5) since that is no longer a man page.

Rename settings net_server to tcpip_socket, max_backends to max_connections
Add --help and --version to postmaster, reformat help output

 when built on its own. UG now builds cleanly.

files to restrict the set of users that can connect to a database
but can still use the pg_shadow password. (You just leave off the
password field in the secondary file.)