hostip: enforce a maximum DNS cache size independent of timeout value

To reduce the damage an application can cause if using -1 or other
ridiculous timeout values and letting the cache live long times.

The maximum number of entries in the DNS cache is now totally
arbitrarily and hard-coded set to 29999.

Closes #11084

docs/libcurl/opts/CURLOPT_DNS_CACHE_TIMEOUT.3

@@ -37,15 +37,23 @@ memory and used for this number of seconds. Set to zero to completely disable
 caching, or set to -1 to make the cached entries remain forever. By default,
 libcurl caches this info for 60 seconds.
 
+We recommend users not to tamper with this option unless strictly necessary.
+If you do, be careful of using large values that can make the cache size grow
+significantly if many different host names are used within that timeout
+period.
+
 The name resolve functions of various libc implementations do not re-read name
 server information unless explicitly told so (for example, by calling
 \fIres_init(3)\fP). This may cause libcurl to keep using the older server even
 if DHCP has updated the server info, and this may look like a DNS cache issue
 to the casual libcurl-app user.
 
-Note that DNS entries have a "TTL" property but libcurl does not use that. This
-DNS cache timeout is entirely speculative that a name will resolve to the same
+DNS entries have a "TTL" property but libcurl does not use that. This DNS
+cache timeout is entirely speculative that a name will resolve to the same
 address for a certain small amount of time into the future.
+
+Since version 8.1.0, libcurl prunes entries from the DNS cache if it excceeds
+30,000 entries no matter which timeout value is used.
 .SH DEFAULT
 60
 .SH PROTOCOLS