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.

Native.cs

@@ -27,6 +27,8 @@
 using System;
 using System.Runtime.InteropServices;
 
+[assembly: DefaultDllImportSearchPathsAttribute(DllImportSearchPath.SafeDirectories)]
+
 namespace Xamarin.Tools.Zip
 {
 	internal class Native
@@ -47,16 +49,16 @@ public struct zip_source_args_seek_t
 
 		public struct zip_stat_t
 		{
-			public UInt64 valid;                 /* which fields have valid values */ 
-			public IntPtr name;                  /* name of the file (char *) */ 
-			public UInt64 index;                 /* index within archive */ 
-			public UInt64 size;                  /* size of file (uncompressed) */ 
-			public UInt64 comp_size;             /* size of file (compressed) */ 
-			public IntPtr mtime;                 /* modification time (time_t) */ 
-			public UInt32 crc;                   /* crc of file data */ 
-			public Int16  comp_method;           /* compression method used */ 
-			public UInt16 encryption_method;     /* encryption method used */ 
-			public UInt32 flags;                 /* reserved for future use */ 
+			public UInt64 valid;                 /* which fields have valid values */
+			public IntPtr name;                  /* name of the file (char *) */
+			public UInt64 index;                 /* index within archive */
+			public UInt64 size;                  /* size of file (uncompressed) */
+			public UInt64 comp_size;             /* size of file (compressed) */
+			public IntPtr mtime;                 /* modification time (time_t) */
+			public UInt32 crc;                   /* crc of file data */
+			public Int16  comp_method;           /* compression method used */
+			public UInt16 encryption_method;     /* encryption method used */
+			public UInt32 flags;                 /* 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)]
 		public static extern int zip_set_file_compression (IntPtr archive, UInt64 index, CompressionMethod comp, UInt32 comp_flags);
-		
+
 		[DllImport (ZIP_LIBNAME, CallingConvention = CallingConvention.Cdecl)]
 		public static extern int zip_file_set_mtime(IntPtr archive, UInt64 index, ulong mtime, UInt32 flags);