url: Add option CURLOPT_RESOLVER_START_FUNCTION

- Add new option CURLOPT_RESOLVER_START_FUNCTION to set a callback that
  will be called every time before a new resolve request is started
  (ie before a host is resolved) with a pointer to backend-specific
  resolver data. Currently this is only useful for ares.

- Add new option CURLOPT_RESOLVER_START_DATA to set a user pointer to
  pass to the resolver start callback.

Closes https://github.com/curl/curl/pull/2311

tests/data/Makefile.inc

@@ -80,7 +80,7 @@ test617 test618 test619 test620 test621 test622 test623 test624 test625 \
 test626 test627 test628 test629 test630 test631 test632 test633 test634 \
 test635 test636 test637 test638 test639 test640 test641 test642 \
 test643 test644 test645 test646 test647 test648 test649 test650 test651 \
-test652 test653 test654 \
+test652 test653 test654 test655 \
 \
 test700 test701 test702 test703 test704 test705 test706 test707 test708 \
 test709 test710 test711 test712 test713 test714 test715 \

tests/data/test655

@@ -0,0 +1,50 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<data>
+HTTP/1.1 200 OK
+Date: Thu, 09 Nov 2010 14:49:00 GMT
+Server: test-server/fake swsclose
+Connection: close
+Content-Type: text/html
+
+hello
+</data>
+<datacheck>
+hello
+</datacheck>
+</reply>
+
+# Client-side
+<client>
+<server>
+http
+</server>
+# tool is what to use instead of 'curl'
+<tool>
+lib655
+</tool>
+
+ <name>
+resolver start callback
+ </name>
+ <command>
+http://%HOSTIP:%HTTPPORT/655
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<errorcode>
+0
+</errorcode>
+</verify>
+</testcase>