AE_ERR -> ANET_ERR in acceptUnixHandler().

No actual changes since the value is the same.
Accept multiple clients per iteration.

When the listening sockets readable event is fired, we have the chance
to accept multiple clients instead of accepting a single one. This makes
Redis more responsive when there is a mass-connect event (for example
after the server startup), and in workloads where a connect-disconnect
pattern is used often, so that multiple clients are waiting to be
accepted continuously.

As a side effect, this commit makes the LOADING, BUSY, and similar
errors much faster to deliver to the client, making Redis more
responsive when there is to return errors to inform the clients that the
server is blocked in an not interruptible operation.
Process events with processEventsWhileBlocked() when blocked.

When we are blocked and a few events a processed from time to time, it
is smarter to call the event handler a few times in order to handle the
accept, read, write, close cycle of a client in a single pass, otherwise
there is too much latency added for clients to receive a reply while the
server is busy in some way (for example during the DB loading).
Test: fixed scripting.tcl test false positive.
hiredis: Update to latest version

This is hiredis f225c276be7fd0646019b51023e3f41566633dfe

This update includes all changes that diverged inside of Redis since
the last update.  This version also allows optional source address
binding for connections which we need for some Sentinel deployments.
hiredis: Apply Redis SDS into hiredis

Redis SDS uses zmalloc instead of the regualr SDS library
which uses regular malloc everywhere.
Add REDIS_BIND_ADDR access macro

We need to access (bindaddr[0] || NULL) in a few places, so centralize
access with a nice macro.
Sentinel: bind source address

Some deployments need traffic sent from a specific address.  This
change uses the same policy as Cluster where the first listed bindaddr
becomes the source address for outgoing Sentinel communication.

Fixes #1667