Use DefaultDllImportSearchPathsAttribute (#66)

Fixes#64
Context: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1139578
Context: https://liquid.microsoft.com/Web/Object/Read/ms.security/Requirements/Microsoft.Security.SystemsADM.10039#guide
The current security guidance is that the [`System.Runtime.InteropServices.DefaultDllImportSearchPathsAttribute`](https://docs.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.defaultdllimportsearchpathsattribute?view=netcore-3.1)
attribute should be placed either on the assembly or on `[DllImport]`
methods, to control and constrain where [`LoadLibraryEx()`](https://docs.microsoft.com/en-us/windows/win32/api/libloaderapi/nf-libloaderapi-loadlibraryexa?redirectedfrom=MSDN)
will look for native libraries.
This commit implements this in the `Native.cs` file. We are using
an assembly level atribute but place it in the area where all the
native calls are maintained.

Author: dellis1972

Native.cs

@@ -27,6 +27,8 @@
 usingSystem;
 usingSystem.Runtime.InteropServices;
 
+[assembly:DefaultDllImportSearchPathsAttribute(DllImportSearchPath.SafeDirectories)]
+
 namespaceXamarin.Tools.Zip
 {
 internalclassNative
@@ -47,16 +49,16 @@ public struct zip_source_args_seek_t
 
 publicstructzip_stat_t
 {
-publicUInt64valid;/* which fields have valid values */
-publicIntPtrname;/* name of the file (char *) */
-publicUInt64index;/* index within archive */
-publicUInt64size;/* size of file (uncompressed) */
-publicUInt64comp_size;/* size of file (compressed) */
-publicIntPtrmtime;/* modification time (time_t) */
-publicUInt32crc;/* crc of file data */
-publicInt16comp_method;/* compression method used */
-publicUInt16encryption_method;/* encryption method used */
-publicUInt32flags;/* reserved for future use */
+publicUInt64valid;/* which fields have valid values */
+publicIntPtrname;/* name of the file (char *) */
+publicUInt64index;/* index within archive */
+publicUInt64size;/* size of file (uncompressed) */
+publicUInt64comp_size;/* size of file (compressed) */
+publicIntPtrmtime;/* modification time (time_t) */
+publicUInt32crc;/* crc of file data */
+publicInt16comp_method;/* compression method used */
+publicUInt16encryption_method;/* encryption method used */
+publicUInt32flags;/* reserved for future use */
 };
 
 [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
@@ -360,7 +362,7 @@ public static int zip_set_file_comment (IntPtr archive, UInt64 index, string com
 
 [DllImport(ZIP_LIBNAME,CallingConvention=CallingConvention.Cdecl)]
 publicstaticexternintzip_set_file_compression(IntPtrarchive,UInt64index,CompressionMethodcomp,UInt32comp_flags);
-
+
 [DllImport(ZIP_LIBNAME,CallingConvention=CallingConvention.Cdecl)]
 publicstaticexternintzip_file_set_mtime(IntPtrarchive,UInt64index,ulongmtime,UInt32flags);