mingw: load system libraries the recommended way

When we access IPv6-related functions, we load the corresponding system
library using the `LoadLibrary()` function, which is not the recommended
way to load system libraries.

In practice, it does not make a difference: the `ws2_32.dll` library
containing the IPv6 functions is already loaded into memory, so
LoadLibrary() simply reuses the already-loaded library.

Still, recommended way is recommended way, so let's use that instead.

While at it, also adjust the code in contrib/ that loads system libraries.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -1686,7 +1686,8 @@ static void ensure_socket_initialization(void)
 			WSAGetLastError());
 
 	for (name = libraries; *name; name++) {
-		ipv6_dll = LoadLibrary(*name);
+		ipv6_dll = LoadLibraryExA(*name, NULL,
+					  LOAD_LIBRARY_SEARCH_SYSTEM32);
 		if (!ipv6_dll)
 			continue;
 

contrib/credential/wincred/git-credential-wincred.c

@@ -75,7 +75,8 @@ static CredDeleteWT CredDeleteW;
 static void load_cred_funcs(void)
 {
 	/* load DLLs */
-	advapi = LoadLibrary("advapi32.dll");
+	advapi = LoadLibraryExA("advapi32.dll", NULL,
+				LOAD_LIBRARY_SEARCH_SYSTEM32);
 	if (!advapi)
 		die("failed to load advapi32.dll");