pgsql: Add a server version check to pg_basebackup and pg_receivexlog.

Add a server version check to pg_basebackup and pg_receivexlog.

These programs don't work against 9.0 or earlier servers, so check that when
the connection is made. That's better than a cryptic error message you got
before.

Also, these programs won't work with a 9.3 server, because the WAL streaming
protocol was changed in a non-backwards-compatible way. As a general rule,
we don't make any guarantee that an old client will work with a new server,
so check that. However, allow a 9.1 client to connect to a 9.2 server, to
avoid breaking environments that currently work; a 9.1 client happens to
work with a 9.2 server, even though we didn't make any great effort to
ensure that.

This patch is for the 9.1 and 9.2 branches, I'll commit a similar patch to
master later. Although this isn't a critical bug fix, it seems safe enough
to back-patch. The error message you got when connecting to a 9.3devel
server without this patch was cryptic enough to warrant backpatching.

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/6980497f7d7f4d17b918a7a433aa904943a4bb97

Modified Files
--------------
src/bin/pg_basebackup/pg_basebackup.c  |   18 ++++++++++++++++++
src/bin/pg_basebackup/pg_receivexlog.c |   18 ++++++++++++++++++
2 files changed, 36 insertions(+), 0 deletions(-)