configure: silence compiler warnings in feature checks, drop duplicates

Silence compiler warnings (200 of them across the main CI workflows):
```
warning curl#2193: null argument provided for parameter marked with attribute "nonnull"
warning: Null pointer passed to 1st parameter expecting 'nonnull' [core.NonNullParamChecker]
warning: Null pointer passed to 2nd parameter expecting 'nonnull' [core.NonNullParamChecker]
warning: argument 1 null where non-null expected [-Wnonnull]
warning: argument 2 null where non-null expected [-Wnonnull]
warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits]
warning: null argument where non-null required (argument 1) [-Wnonnull]
```

Also drop `if ... can be linked` feature checks that were identical to
`if ... is compilable` checks, for:
`closesocket`, `ioctlsocket`, `socket`, `freeaddrinfo`, `getaddrinfo`,
`gethostname`, `getpeername`, `getsockname`,
`CloseSocket` (AmigaOS), `IoctlSocket` (AmigaOS).
Another option is to really do the link checks. But, if they weren't
missed so far, it seems safer to drop than risk a detection failure,
as was the case with AmigaOS functions while working on this PR.

There remain 22 `-Wnonnull` warnings in `gethostbyname_r()`,
`getpeername()` `getsockname()`. Most of the rest is necessary for
detection, or originate from autotools and CMake detection code
templates. Some still fixable, like duplicate libs.

Follow-up to ca2f49d curl#16287
Closes curl#16377

acinclude.m4

@@ -689,7 +689,8 @@ AC_DEFUN([CURL_CHECK_FUNC_SEND], [
       #endif
       #endif
     ]],[[
-      send(0, 0, 0, 0);
+      char s[] = "";
+      send(0, (void *)s, 0, 0);
     ]])
   ],[
     AC_MSG_RESULT([yes])

configure.ac

@@ -1757,7 +1757,7 @@ AS_HELP_STRING([--disable-ipv6],[Disable IPv6 support]),
       {
         int s = (int)sizeof(struct sockaddr_in6);
         (void)s;
-        return socket(AF_INET6, SOCK_STREAM, 0) < 0;
+        return socket(AF_INET6, SOCK_STREAM, 0) > 0;
       }
     ]])
   ],