add two env vars to pass password/keyphrase (#9)

MSCP_SSH_AUTH_PASSWORD passes a password, and MSCP_SSH_AUTH_PASSPHRASE
passes a passphrase for publickey auth. They enable avoiding interactive
password input. Test cases are also added.

src/main.c

@@ -344,6 +344,10 @@ int main(int argc, char **argv)
 		}
 	}
 
+	s.password = getenv(ENV_SSH_AUTH_PASSWORD);
+	s.passphrase = getenv(ENV_SSH_AUTH_PASSPHRASE);
+
+
 	if (argc - optind < 2) {
 		/* mscp needs at lease 2 (src and target) argument */
 		usage(false);