archive_xlog: make archiving fail more strictly

Before this patch, only a failure on all targets return an error to
PostgreSQL. This is not good when a slave goes down then up, because it
would get a hole in the WAL files chain, the admin would less easily
find what happened.

Now, if a copy fails, it is recorded and the process continues on the
other targets. When done, the script give return code 1 to PostgreSQL
to make the master retry later.
fix tablespace listing query to support 9.2

backup_pitr uses a query on pg_tablespace to list the defined
tablespaces. In 9.2, the spclocation column of pg_tablespace has been
removed, the location can be retrieved using pg_tablespace_location().
So we check the version and issu the proper query.
Prepare 1.2 release