tidy-up: delete, comment or scope C macros reported unused

To reduce the number `-Wunused-macro` compiler warnings:
- delete unused macros.
- comment out unused macro that are part of a set.
- move macros into the scope they are used.

This may be useful to enable by default, but there are tricky cases that
I didn't manage to fix and paused the effort. E.g. internal features
checks in `openssl.c`. There is more, once those are fixed.

Closes #16279

tests/server/mqttd.c

@@ -65,12 +65,6 @@
 #ifdef USE_WINSOCK
 #undef  EINTR
 #define EINTR    4 /* errno.h value */
-#undef  EAGAIN
-#define EAGAIN  11 /* errno.h value */
-#undef  ENOMEM
-#define ENOMEM  12 /* errno.h value */
-#undef  EINVAL
-#define EINVAL  22 /* errno.h value */
 #endif
 
 #define DEFAULT_PORT 1883 /* MQTT default port */
@@ -86,15 +80,11 @@
 #define MQTT_MSG_CONNECT    0x10
 #define MQTT_MSG_CONNACK    0x20
 #define MQTT_MSG_PUBLISH    0x30
-#define MQTT_MSG_PUBACK     0x40
+/* #define MQTT_MSG_PUBACK     0x40 */
 #define MQTT_MSG_SUBSCRIBE  0x82
 #define MQTT_MSG_SUBACK     0x90
 #define MQTT_MSG_DISCONNECT 0xe0
 
-#define MQTT_CONNACK_LEN 4
-#define MQTT_SUBACK_LEN 5
-#define MQTT_CLIENTID_LEN 12 /* "curl0123abcd" */
-
 struct configurable {
   unsigned char version; /* initial version byte in the request must match
                             this */

tests/server/rtspd.c

@@ -68,7 +68,6 @@ static const char *ipv_inuse = "IPv4";
 static int serverlogslocked = 0;
 
 #define REQBUFSIZ 150000
-#define REQBUFSIZ_TXT "149999"
 
 static long prevtestno = -1;    /* previous test number we served */
 static long prevpartno = -1;    /* previous part number we served */

tests/server/socksd.c

@@ -86,12 +86,6 @@
 #ifdef USE_WINSOCK
 #undef  EINTR
 #define EINTR    4 /* errno.h value */
-#undef  EAGAIN
-#define EAGAIN  11 /* errno.h value */
-#undef  ENOMEM
-#define ENOMEM  12 /* errno.h value */
-#undef  EINVAL
-#define EINVAL  22 /* errno.h value */
 #endif
 
 #define DEFAULT_PORT 8905