Sentinel: Reject config from STDIN

Sentinel needs to die with a more accurate error message
when attempted to open a config from STDIN.

See: https://groups.google.com/forum/#!topic/redis-db/sYx7VNMWaNM
Stop tests from leaving a black background

Uses ANSI "default background" color code after closing tests
so any non-black terminals don't remain polluted.

Fixes #1649
Closes #1912
Use correct github url to find commands.json

Once this is merged:
  - merge the latest changes to commands.json in antirez/redis-doc
  - re-run: utils/generate-command-help.rb > src/help.h

Then we'll have nice and easy tab-completed help in redis-cli again.

Closes #1909
Sentinel: fix bufsize to support IPv6 address

Closes #1914
Rename two 'buf' vars to 'ip' for better clarity

Clearly ip[32] is wrong, but it's less clear that buf[32] was wrong
without further reading.
redis-benchmark: add auth option

Closes #1097
redis-benchmark: Remove unused buffer

This just deletes old code that didn't get removed when
logic changed.  We were setting offsets that never
got read anywhere.

Since clients are now just cloned, we don't need to track
per-client buffer offsets anywhere because they are all
the same from the original client.