Allow AUTH while loading the DB in memory.

While Redis is loading the AOF or RDB file in memory only a subset of
commands are allowed. This commit adds AUTH to this subset.
Don't segfault on unbalanced quotes.
sdssplitargs(): now returns NULL only on error.

An empty input string also resulted into the function returning NULL
making it harder for the caller to distinguish between error and empty
string without checking the original input string length.
sdssplitargs(): on error set *argc to 0.

This makes programs not checking the return value for NULL much safer
since with this change:

1) It is still possible to iterate the zero-length result without
crashes.
2) sdssplitargs_free will work against NULL and 0 count.
More specific error message in loadServerConfigFromString().
sds.c: sdssplitargs_free() removed as it was a duplicate.
redis-cli: use sdsfreesplitres() instead of hand-coding it.
Handle a non-impossible empty argv in loadServerConfigFromString().

Usually this does not happens since we trim for " \t\r\n", but if there
are other chars that return true with isspace(), we may end with an
empty argv. Better to handle the condition in an explicit way.
Add a warning about command renaming in redis.conf.
API to lookup commands with their original name.

A new server.orig_commands table was added to the server structure, this
contains a copy of the commant table unaffected by rename-command
statements in redis.conf.

A new API lookupCommandOrOriginal() was added that checks both tables,
new first, old later, so that rewriteClientCommandVector() and friends
can lookup commands with their new or original name in order to fix the
client->cmd pointer when the argument vector is renamed.

This fixes the segfault of issue #986, but does not fix a wider range of
problems resulting from renaming commands that actually operate on data
and are registered into the AOF file or propagated to slaves... That is
command renaming should be handled with care.
Removed useless "return" statements in pubsub.c

(original commit message edited)