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/util.h

@@ -51,14 +51,27 @@ enum {
 
 char *data_to_hex(char *data, size_t len);
 void logmsg(const char *msg, ...) CURL_PRINTF(1, 2);
+void loghex(unsigned char *buffer, ssize_t len);
+unsigned char byteval(char *value);
 
 #define SERVERLOGS_LOCKDIR "lock"  /* within logdir */
 
-/* global variable, where to find the 'data' dir */
-extern const char *path;
-
-/* global variable, log file name */
-extern const char *serverlogfile;
+/* global variables */
+extern const char *path;  /* where to find the 'data' dir */
+extern const char *pidname;
+extern const char *portname;
+extern const char *serverlogfile;  /* log file name */
+extern int serverlogslocked;
+extern const char *configfile;
+extern const char *logdir;
+extern char loglockfile[256];
+#ifdef USE_IPV6
+extern bool use_ipv6;
+#endif
+extern const char *ipv_inuse;
+extern unsigned short server_port;
+extern const char *socket_type;
+extern int socket_domain;
 
 #ifdef _WIN32
 int win32_init(void);