hostip: cache negative name resolves

Hold them for half the normal lifetime. Helps when told to transfer N
URLs in quick succession that all use the same non-resolving hostname.

Done by storing a DNS entry with a NULL pointer for 'addr'.

Previously an attempt was made in #12406 by Björn Stenberg that was
ultimately never merged.

Closes #18157

tests/data/Makefile.am

@@ -252,7 +252,7 @@ test2064 test2065 test2066 test2067 test2068 test2069 test2070 test2071 \
 test2072 test2073 test2074 test2075 test2076 test2077 test2078 test2079 \
 test2080 test2081 test2082 test2083 test2084 test2085 test2086 test2087 \
 test2088 test2089 \
-test2100 test2101 test2102 test2103 \
+test2100 test2101 test2102 test2103 test2104 \
 \
 test2200 test2201 test2202 test2203 test2204 test2205 \
 \

tests/data/test2104

@@ -0,0 +1,49 @@
+<testcase>
+<info>
+<keywords>
+DNS cache
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<dns>
+</dns>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+dns
+</server>
+<features>
+override-dns
+</features>
+<name>
+Get three URLs with bad host name - cache
+</name>
+<setenv>
+CURL_DNS_SERVER=127.0.0.1:%DNSPORT
+</setenv>
+<command>
+http://examplehost.example/%TESTNUMBER http://examplehost.example/%TESTNUMBER http://examplehost.example/%TESTNUMBER
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+# curl: (6) Could not resolve host: examplehost.example
+<errorcode>
+6
+</errorcode>
+
+# Ignore HTTPS requests here
+<dns host="examplehost.example QTYPE A">
+QNAME examplehost.example QTYPE A
+QNAME examplehost.example QTYPE AAAA
+</dns>
+</verify>
+</testcase>

tests/data/test655

@@ -36,7 +36,7 @@ lib%TESTNUMBER
 resolver start callback
 </name>
 <command>
-http://%HOSTIP:%HTTPPORT/%TESTNUMBER
+http://failthis/%TESTNUMBER http://%HOSTIP:%HTTPPORT/%TESTNUMBER
 </command>
 </client>