This commit fixes issue #875 that was caused by the following events:

1) There is an active child doing BGSAVE.
2) flushall is called (or any other condition that makes Redis killing
the saving child process).
3) An error is sensed by Redis as the child exited with an error (killed
by a singal), that stops accepting write commands until a BGSAVE happens
to be executed with success.

Whitelisting SIGUSR1 and making sure Redis always uses this signal in
order to kill its own children fixes the issue.

The previous behavior was to return -1 if:

1) Existing key but without an expire set.
2) Non existing key.

Now the second case is handled in a different, and TTL will return -2
if the key does not exist at all.

PTTL follows the same behavior as well.

Unfortunately we had still the lame atoi() without any error checking in
place, so "SELECT foo" would work as "SELECT 0". This was not an huge
problem per se but some people expected that DB can be strings and not
just numbers, and without errors you get the feeling that they can be
numbers, but not the behavior.

Now getLongFromObjectOrReply() is used as almost everybody else across
the code, generating an error if the number is not an integer or
overflows the long type.

Thanks to @mipearson for reporting that on Twitter.

Every matched key in a KEYS call is checked for expiration. When the key
is set to expire, the call to `getExpire` will assert that the key also
exists in the main dictionary. This in turn causes a rehashing step to
be executed. Rehashing a dictionary when there is an iterator active may
result in the iterator emitting duplicate entries, or not emitting some
entries at all. By using a safe iterator, the rehash step is omitted.

expireGenericCommand(): better variable names and a top-comment that describes the function's behavior.

Use less memory when emitting the protocol, by using more shared objects for commonly emitted parts of the protocol.

setKey(): call the higher level wrapper setModifiedKey() instead of touchWatchedKey() even if currently they are exactly the same.

New script timeout semantics and SCRIPT KILL implemented. SHUTDOWN NOSAVE and SHUTDOWN SAVE implemented.

high resolution expires API modified to use separated commands. AOF transation to PEXPIREAT of all the expire-style commands fixed.

Initial support for key expire times with millisecond resolution. RDB version is now 3, new opcoded added for high resolution times. Redis is still able to correctly load RDB version 2. Tests passing but still a work in progress. API to specify milliseconds expires still missing, but the precision of normal expires is now already improved and working.

FLUSHALL now prevents rdbSave() from resetting the dirty counter, so that the command will get replicated and put inside the AOF. This fixes issue #142

don't process EXPIRE with negative TTL or EXPIREAT with time in the past if we are a slave too (see http://groups.google.com/group/redis-db/browse_thread/thread/5a931fefb88b16d5). Also propagate it as DEL.

They will be expired (and a DEL will be logged) after the loading is done
instead.

Fixed semantics of CLUSTER SETSLOT, SELECT now only denied in cluster mode if selected DB is not 0 so that MIGRATE still works well.