Add GUC_REPORT to server_encoding, integer_datetimes

This adds GUC_REPORT to server_encoding and integer_datetimes so they
are reported in the V3 protocol startup packet. Also some related doc
updates.

Rationale:

1) server_encoding is useful to allow clients to detect bad
server/client encoding pairs. The one that bites JDBC regularly is
client_encoding = UNICODE with server_encoding = SQL_ASCII. I'd prefer
for the server to reject that combination entirely, but failing that
clients can implement such a policy themselves by inspecting
server_encoding when establishing a connection.

2) integer_datetimes affects the binary representation of date/time
types. A client that wants to use binary parameters or resultsets that
involve those types needs to know the value of this setting before
dealing with the binary data.

In both cases, it's good to have it in the startup packet to avoid an
extra round-trip on connection establishment.

Any chance that this can go into 8.0?

-O