multi: add CURLMOPT_NETWORK_CHANGED to signal network changed

New multi option CURLMOPT_NETWORK_CHANGED with a long bitmask value:

- CURLM_NWCOPT_CLEAR_CONNS: do not reuse existing connections, close all
  idle connections.

- CURLM_NWCOPT_CLEAR_DNS: clear the multi's DNS cache.

All other bits reserved for future extensions.

Fixes #17225
Reported-by: ウさん
Closes #17613

tests/data/Makefile.am

@@ -273,7 +273,7 @@ test3000 test3001 test3002 test3003 test3004 test3005 test3006 test3007 \
 test3008 test3009 test3010 test3011 test3012 test3013 test3014 test3015 \
 test3016 test3017 test3018 test3019 test3020 test3021 test3022 test3023 \
 test3024 test3025 test3026 test3027 test3028 test3029 test3030 test3031 \
-test3032 \
+test3032 test3033 \
 \
 test3100 test3101 test3102 test3103 test3104 test3105 \
 \

tests/data/test3033

@@ -0,0 +1,53 @@
+<testcase>
+<info>
+<keywords>
+curl_easy_setopt
+connection reuse
+libtest
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<data>
+HTTP/1.1 200 OK
+Content-Length: 6
+
+-foo-
+</data>
+<datacheck>
+[0] no network change
+-foo-
+[1] signal network change
+-foo-
+[2] no network change
+-foo-
+</datacheck>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+http
+</server>
+<name>
+CURLOPT_FRESH_CONNECT=2
+</name>
+<tool>
+lib%TESTNUMBER
+</tool>
+<command>
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<errorcode>
+0
+</errorcode>
+</verify>
+</testcase>