Fix config.h endianess detection to work on Linux / PPC64.

Config.h performs endianess detection including OS-specific headers to
define the endianess macros, or when this is not possible, checking the
processor type via ifdefs.

Sometimes when the OS-specific macro is included, only __BYTE_ORDER is
defined, while BYTE_ORDER remains undefined. There is code at the end of
config.h endianess detection in order to define the macros without the
underscore, but it was not working correctly.

This commit fixes endianess detection fixing Redis on Linux / PPC64 and
possibly other systems.
Define _XOPEN_SOURCE appropriately on NetBSD.
Added missing license and copyright in deps/hiredis.
CONTRIBUTING updated with request to add BSD license.
Fix overflow in mstime() in redis-cli and benchmark.

The problem does not exist in the Redis server implementation of mstime()
but is only limited to redis-cli and redis-benchmark.

Thix fixes issue #839.
ae.c: set errno when error is not a failing syscall.

In this way the caller is able to perform better error checking or to
use strerror() without the risk of meaningless error messages being
displayed.
Better error reporting when fd event creation fails.
Multiple fixes for EVAL (issue #872).

1) The event handler was no restored after a timeout condition if the
   command was eventually executed with success.
2) The command was not converted to EVAL in case of errors in the middle
   of the execution.
3) Terrible duplication of code without any apparent reason.
Test: added regression for issue #872.
Comment in the call() function clarified a bit.
Redis 2.6.8
Makefile.dep updated.
Merge upstream Redis 2.6.8.