curl_easy_duphandle() was not properly duping the ares channel. The
ares_dup() function was introduced in c-ares 1.6.0 so by starting to use
this function we also raise the bar and require c-ares >= 1.6.0
(released Dec 9, 2008) for such builds.

Reported by: Ning Dong
Bug: http://curl.haxx.se/mail/lib-2010-08/0318.html

It's not quite fair to list TFTP is a "crappy" member of the
libcurl family so I removed its mentioning.

removed trailing whitespace

renamed generated config.h to curl_config.h in order to avoid clashes when libcurl is used with other projects which also have a config.h.

in regard to C standard, third party libraries and operating systems etc.

internally