resolved: use strv_extend_with_size() to avoid slow parsing of /etc/hosts

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=43942 is a simple case
where a repeated entry generates a timeout. I didn't import that case, but
generated a simpler one by hand.

$ time build/fuzz-etc-hosts test/fuzz/fuzz-etc-hosts/timeout-many-entries
test/fuzz/fuzz-etc-hosts/timeout-many-entries... ok
build/fuzz-etc-hosts test/fuzz/fuzz-etc-hosts/timeout-many-entries  3.17s (old)
  ↓
build/fuzz-etc-hosts test/fuzz/fuzz-etc-hosts/timeout-many-entries  0.11s (new)

I considered simply disallowing too many aliases. E.g. microsoft appearently
sometimes ignores entries after the ninth [1], and other systems set stringent
limits [2,3], but the recommended way to get around that is to simply use more
lines (as is done in the sample), so this wouldn't change anything.

Even if we cannot put all those names in a reply packet, the resolution from
the alias to the address should work. I think cases where people define lots
and lots of aliases through some programmatic interface is realistic, for
example for a blocklist, and such a file shouldn't bring resolved down to its
knees.

[1] https://superuser.com/questions/932112/is-there-a-maximum-number-of-hostname-aliases-per-line-in-a-windows-hosts-file
[2] https://library.netapp.com/ecmdocs/ECMP1516135/html/GUID-C6F3B6D1-232D-44BB-A76C-3304C19607A3.html
[3] https://www.ibm.com/docs/en/zos/2.1.0?topic=optional-creating-etchosts

src/resolve/resolved-etc-hosts.c

@@ -127,7 +127,7 @@ static int parse_line(EtcHosts *hosts, unsigned nr, const char *line) {
                         continue;
                 }
 
-                r = strv_extend(&item->names, name);
+                r = strv_extend_with_size(&item->names, &item->n_names, name);
                 if (r < 0)
                         return log_oom();
 
@@ -391,7 +391,7 @@ int manager_etc_hosts_lookup(Manager *m, DnsQuestion* q, DnsAnswer **answer) {
                 }
 
                 if (found_ptr) {
-                        r = dns_answer_reserve(answer, strv_length(item->names));
+                        r = dns_answer_reserve(answer, item->n_names);
                         if (r < 0)
                                 return r;
 

src/resolve/resolved-etc-hosts.h

@@ -9,6 +9,7 @@ typedef struct EtcHostsItem {
         struct in_addr_data address;
 
         char **names;
+        size_t n_names;
 } EtcHostsItem;
 
 typedef struct EtcHostsItemByName {