[Mono.Security]: minor `CryptoConvert` changes to make it more linker-friendly.

* Add new internal `CryptoConvert.TryImportCapiPrivateKeyBlob()`
  method that's using `RSAManaged` directly to make it more
  linker-friendly.

* Use it in `AssemblyName.IsPublicKeyValid`.

mcs/class/Mono.Security/Mono.Security.Cryptography/CryptoConvert.cs

@@ -77,6 +77,24 @@ namespace Mono.Security.Cryptography {
 			return null;
 		}
 
+#if INSIDE_CORLIB
+		static internal bool TryImportCapiPrivateKeyBlob (byte[] blob, int offset)
+		{
+			try {
+				var rsap = GetParametersFromCapiPrivateKeyBlob (blob, offset);
+				// Since we are only checking whether this throws an exception and
+				// not actually returning the `RSA` object, we can use `RSAManaged`
+				// here because that's what the `RSACryptoServiceProvider` implementation
+				// does internally.
+				var rsa = new RSAManaged ();
+				rsa.ImportParameters (rsap);
+				return true;
+			} catch (CryptographicException) {
+				return false;
+			}
+		}
+#endif
+
 		// convert the key from PRIVATEKEYBLOB to RSA
 		// http://msdn.microsoft.com/library/default.asp?url=/library/en-us/security/Security/private_key_blobs.asp
 		// e.g. SNK files, PVK files
@@ -86,6 +104,38 @@ namespace Mono.Security.Cryptography {
 		}
 
 		static public RSA FromCapiPrivateKeyBlob (byte[] blob, int offset) 
+		{
+			RSAParameters rsap = GetParametersFromCapiPrivateKeyBlob (blob, offset);
+
+#if INSIDE_CORLIB && MOBILE
+			RSA rsa = RSA.Create ();
+			rsa.ImportParameters (rsap);
+#else
+			RSA rsa = null;
+			try {
+				rsa = RSA.Create ();
+				rsa.ImportParameters (rsap);
+			}
+			catch (CryptographicException ce) {
+				// this may cause problem when this code is run under
+				// the SYSTEM identity on Windows (e.g. ASP.NET). See
+				// http://bugzilla.ximian.com/show_bug.cgi?id=77559
+				try {
+					CspParameters csp = new CspParameters ();
+					csp.Flags = CspProviderFlags.UseMachineKeyStore;
+					rsa = new RSACryptoServiceProvider (csp);
+					rsa.ImportParameters (rsap);
+				}
+				catch {
+					// rethrow original, not the later, exception if this fails
+					throw ce;
+				}
+			}
+#endif
+			return rsa;
+		}
+
+		static RSAParameters GetParametersFromCapiPrivateKeyBlob (byte[] blob, int offset)
 		{
 			if (blob == null)
 				throw new ArgumentNullException ("blob");
@@ -161,37 +211,10 @@ namespace Mono.Security.Cryptography {
 					Buffer.BlockCopy (blob, pos, rsap.D, 0, byteLen);
 					Array.Reverse (rsap.D);
 				}
-			}
-			catch (Exception e) {
+				return rsap;
+			} catch (Exception e) {
 				throw new CryptographicException ("Invalid blob.", e);
 			}
-
-#if INSIDE_CORLIB && MOBILE
-			RSA rsa = RSA.Create ();
-			rsa.ImportParameters (rsap);
-#else
-			RSA rsa = null;
-			try {
-				rsa = RSA.Create ();
-				rsa.ImportParameters (rsap);
-			}
-			catch (CryptographicException ce) {
-				// this may cause problem when this code is run under
-				// the SYSTEM identity on Windows (e.g. ASP.NET). See
-				// http://bugzilla.ximian.com/show_bug.cgi?id=77559
-				try {
-					CspParameters csp = new CspParameters ();
-					csp.Flags = CspProviderFlags.UseMachineKeyStore;
-					rsa = new RSACryptoServiceProvider (csp);
-					rsa.ImportParameters (rsap);
-				}
-				catch {
-					// rethrow original, not the later, exception if this fails
-					throw ce;
-				}
-			}
-#endif
-			return rsa;
 		}
 
 		static public DSA FromCapiPrivateKeyBlobDSA (byte[] blob)