mention asynchronous DNS lookups enhancements

Restore normal operation:

  c-ares is only enabled when specifically requested.

  Consequently, c-ares default setting is disabled.

fix c-ares assumed check being skipped

fix compiler warning

fix LDFLAGS preservation in CURL_CHECK_LIB_ARES

is now a very long time ago

  proxy that cannot be resolved when using c-ares. This matches the behaviour
  when not using c-ares.

fix compiler warning

HAVE_GETADDRINFO_THREADSAFE definition for non-configure win32 builds

add inclusion of curl_memory.h

adjust preprocessor symbol definition check relative to resolver specialty

PKTSIZE might have been already defined in arpa/tftp.h

from hostip.h to setup.h in order to allow proper inclusion in any file.

This represents no functional change at all in which resolver is used,
everything still works as usual, internally and externally there is no
difference in behavior.

also says so. I also made the list of protocols get sorted.

s/RTPDATA/INTERLEAVEDATA/