1) Don't accept maxclients set to < 0
2) Allow maxclients < 1024, it is useful for testing.

After considering the interaction between ability to delcare globals in
scripts using the 'global' function, and the complexities related to
hanlding replication and AOF in a sane way with globals AND ability to
turn protection On and Off, we reconsidered the design. The new design
makes clear that there is only one good way to write Redis scripts, that
is not using globals. In the rare cases state must be retained across
calls a Redis key can be used.

It is now possible to enable/disable RDB checksum computation from redis.conf or via CONFIG SET/GET. Also CONFIG SET support added for rdbcompression as well.

This new field counts all the times Redis is configured with AOF enabled and
fsync policy 'everysec', but the previous fsync performed by the
background thread was not able to complete within two seconds, forcing
Redis to perform a write against the AOF file while the fsync is still
in progress (likely a blocking operation).

This commit introduces support for read only slaves via redis.conf and CONFIG GET/SET commands. Also various semantical fixes are implemented here:

1) MULTI/EXEC with only read commands now work where the server is into a state where writes (or commands increasing memory usage) are not allowed. Before this patch everything inside a transaction would fail in this conditions.

2) Scripts just calling read-only commands will work against read only
slaves, when the server is out of memory, or when persistence is into an
error condition. Before the patch EVAL always failed in this condition.

Removed handling of deprecated hash-max-zipmap-entries nad hash-map-zipmap-value. Pieter is too good with users ;). Better to have them switch to a saner configuration ASAP after the 2.6 upgrade.

By default Redis refuses writes with an error if the latest BGSAVE failed (and at least one save point is configured). However people having good monitoring systems may prefer a server that continues to work, since they are notified that there are problems by their monitoring systems. This commit implements the ability to turn the feature on or off via redis.conf and CONFIG SET.

Client output buffer limits: configuration of parameters for the different classes of clients implemented.

It is now posible to flush all the previous saving points in redis.conf by using a save directive with a single empty string argument, like it happens for CONFIG SET save.

AOF fileds in the global server state, and define names, renamed with more consistent names. More work to do.

It is now possible to disable password authentication via CONFIG SET. Thanks to Jan Oberst. This closes issue #171.

Added a configuration directive to allow a user to specify the
permissions to be granted to the Unix socket file.  I followed
the format Pieter and Salvatore discusses in issue #85 (
https://github.com/antirez/redis/issues/85).