CONFIG REWRITE: don't wipe unknown options.

With this commit options not explicitly rewritten by CONFIG REWRITE are
not touched at all. These include new options that may not have support
for REWRITE, and other special cases like rename-command and include.
CONFIG REWRITE: old development comments removed.
CONFIG REWRITE: don't throw some options on config rewrite

Those options will be thrown without this patch:
  include, rename-command, min-slaves-to-write, min-slaves-max-lag,
appendfilename.
CONFIG REWRITE: no special handling or include and rename-command.

CONFIG REWRITE is now wiser and does not touch what it does not
understand inside redis.conf.
Clarify include directive behavior in example redis.conf.