ip_quadruple/proxy: make port uint16_t

Make `port` member in these struct of type `uint16_t`.

add `uint8_t transport` to `struct ip_quadruple

Define TRNSPRT_NONE as 0. By assigning a valid transport only on a
successful connection, it is clear when the ip_quadruple members are
valid. Also, for transports not involving ports, the getinfos for
`CURLINFO_PRIMARY_PORT` and `CURLINFO_LOCAL_PORT` will now always return
-1.

Make all `transport` members and parameters of type `uint8_t`.

Document the return value of `CURLINFO_LOCAL_PORT` and
`CURLINFO_PRIMARY_PORT` in this regard. Add tests that writeout stats
report ports correctly.

Closes #19708

tests/unit/unit1607.c

@@ -141,7 +141,7 @@ static CURLcode test_unit1607(const char *arg)
     addr = dns ? dns->addr : NULL;
 
     for(j = 0; j < addressnum; ++j) {
-      int port = 0;
+      uint16_t port = 0;
       char ipaddress[MAX_IPADR_LEN] = {0};
 
       if(!addr && !tests[i].address[j])

tests/unit/unit1609.c

@@ -143,7 +143,7 @@ static CURLcode test_unit1609(const char *arg)
     addr = dns ? dns->addr : NULL;
 
     for(j = 0; j < addressnum; ++j) {
-      int port = 0;
+      uint16_t port = 0;
       char ipaddress[MAX_IPADR_LEN] = {0};
 
       if(!addr && !tests[i].address[j])

tests/unit/unit2600.c

@@ -110,7 +110,7 @@ static int test_idx;
 struct cf_test_ctx {
   int idx;
   int ai_family;
-  int transport;
+  uint8_t transport;
   char id[16];
   struct curltime started;
   timediff_t fail_delay_ms;
@@ -166,7 +166,7 @@ static CURLcode cf_test_create(struct Curl_cfilter **pcf,
                                struct Curl_easy *data,
                                struct connectdata *conn,
                                const struct Curl_addrinfo *ai,
-                               int transport)
+                               uint8_t transport)
 {
   static const struct Curl_cftype cft_test = {
     "TEST",