[net] drop direct hooking into wininet DLL and use delay-loading instead

* Now that we can delay-load DLLs for both MinGW and MSVC, we can remove the
  cumbersome direct DLL hooks into wininet.dll (which is vulnerable to side
  loading when not delay-loaded) and revert to using direct API calls instead.
* This reverts part of e1d864f755.
* Also attempt to silence a Coverity warning.

src/iso.c

@@ -1580,7 +1580,7 @@ BOOL DumpFatDir(const char* path, int32_t cluster)
 	}
 
 	do {
-		// coverity[-taint_source]
+		// coverity[tainted_data_return]
 		dirpos.cluster = libfat_dumpdir(lf_fs, &dirpos, &diritem);
 		if (dirpos.cluster >= 0) {
 			name = wchar_to_utf8(diritem.name);