redis-cli --bigkeys: don't crash with empty DBs.
Set default for stop_writes_on_bgsave_err in initServerConfig().

It was placed for error in initServer() that's called after the
configuation is already loaded, causing issue #1000.
Abort when opening the RDB file results in an error other than ENOENT.

This fixes cases where the RDB file does exist but can't be accessed for
any reason. For instance, when the Redis process doesn't have enough
permissions on the file.
Merge pull request #1001 from djanowski/fatal-errors-rdb-load

Abort when opening the RDB file results in an error other than ENOENT.
rdbLoad(): rework code to save vertical space.