Fdset target is used when we want to write an RDB file directly to
slave's sockets. In this setup as long as there is a single slave that
is still receiving our payload, we want to continue sennding instead of
aborting. However rio calls should abort of no FD is ok.

Also we want the errors reported so that we can signal the parent who is
ok and who is broken, so there is a new set integers with the state of
each fd. Zero is ok, non-zero is the errno of the failure, if avaialble,
or a generic EIO.

We need to remember what is the saving strategy of the current RDB child
process, since the configuration may be modified at runtime via CONFIG
SET and still we'll need to understand, when the child exists, what to
do and for what goal the process was initiated: to create an RDB file
on disk or to write stuff directly to slave's sockets.

During the replication full resynchronization process, the RDB file is
transfered from the master to the slave. However there is a short
preamble to send, that is currently just the bulk payload length of the
file in the usual Redis form $..length..<CR><LF>.

This preamble used to be sent with a direct write call, assuming that
there was alway room in the socket output buffer to hold the few bytes
needed, however this does not scale in case we'll need to send more
stuff, and is not very robust code in general.

This commit introduces a more general mechanism to send a preamble up to
2GB in size (the max length of an sds string) in a non blocking way.

😄

Closes #2034

Closes #2029

Closes #2002

Closes #1960

Closes #1938

Some language in the comment was difficult
to understand, so this commit: clarifies wording, removes
unnecessary words, and relocates some dependent clauses
closer to what they actually describe.

I also tried to break up longer chains of thought
(if X, then Y, and Q, and also F, so obviously M)
into more manageable chunks for ease of understanding.

Closes #1739

Closes #1523

  - Remove trailing newlines from redis.conf
  - Fix comment misspelling
  - Clarifies zipEncodeLength usage and a C API mention (#1243, #1242)
  - Fix cluster typos (inspired by @papanikge #1507)
  - Fix rewite -> rewrite in a few places (inspired by #682)

Closes #1243, #1242, #1507

Closes #1513

Closes #1441

Closes #1386

Closes #1373

Closes #1351

Closes #1871

error != success; and 0 != number of bytes written

Closes #1806

Closes #1713

Closes #1682

Closes #1544

Closes #1537

The core linenoise code was being backported, but not
the README or example.  It's less confusing for users
if everything matches across directories.

Fix inspired by @thrig

Closes #1872

Closes #1897