tests/server: round of tidy-ups (part 2)

General tidy-ups, to identify and reduce duplications and potential
issues, while also making the server modules compile as a single binary.

- ensure unique symbols and no shadowing across server sources, by
  renaming variables.
- move globals common to multiple servers into shared `util` module.
- drop constants with a single use.
- undef macro before re-using them across server sources.
- move common functions into shared `util` module.
- drop redundant static declarations.
- disable IPv6 code when built without IPv6.
- start syncing the 3 almost identical copies of `sockdaemon` function.
- drop unused `timeval.h` header.
- drop `poll()` from `wait_ms()`, for macOS, following an earlier core
  update.
  Follow-up to c72cefea0f #15096

Follow-up to 9213e4e497 #16525
Cherry-picked from #15000
Closes #16609

tests/server/resolve.c

@@ -50,9 +50,6 @@
 /* include memdebug.h last */
 #include "memdebug.h"
 
-static bool use_ipv6 = FALSE;
-static const char *ipv_inuse = "IPv4";
-
 int main(int argc, char *argv[])
 {
   int arg = 1;
@@ -71,14 +68,21 @@ int main(int argc, char *argv[])
       return 0;
     }
     else if(!strcmp("--ipv6", argv[arg])) {
+#if defined(CURLRES_IPV6)
       ipv_inuse = "IPv6";
       use_ipv6 = TRUE;
       arg++;
+#else
+      puts("IPv6 support has been disabled in this program");
+      return 1;
+#endif
     }
     else if(!strcmp("--ipv4", argv[arg])) {
       /* for completeness, we support this option as well */
       ipv_inuse = "IPv4";
+#if defined(CURLRES_IPV6)
       use_ipv6 = FALSE;
+#endif
       arg++;
     }
     else {
@@ -127,13 +131,8 @@ int main(int argc, char *argv[])
       freeaddrinfo(ai);
   }
 #else
-  if(use_ipv6) {
-    puts("IPv6 support has been disabled in this program");
-    return 1;
-  }
-  else {
-    /* gethostbyname() resolve */
-    struct hostent *he;
+  {
+    struct hostent *he;  /* gethostbyname() resolve */
 
 #ifdef __AMIGA__
     he = gethostbyname((unsigned char *)host);