instproxy: Use synchronous mode when callbacks are NULL, as documented

Synchronous modes for `instproxy_install`, `instproxy_upgrade`, `instproxy_uninstall`, `instproxy_archive`, `instproxy_restore`, and `instproxy_remove_archive` appear to be ignoring the documented behavior that “If NULL is passed, this function will run synchronously”.

This commit fixes the advertised behavior.

src/installation_proxy.c

diff --git a/src/installation_proxy.c b/src/installation_proxy.c
index eedddc7..b13abf9 100644
--- a/src/installation_proxy.c
+++ b/src/installation_proxy.c
@@ -660,7 +660,7 @@
 		plist_dict_set_item(command, "ClientOptions", plist_copy(client_options));
 	plist_dict_set_item(command, "PackagePath", plist_new_string(pkg_path));
 
-	res = instproxy_perform_command(client, command, INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
+	res = instproxy_perform_command(client, command, status_cb == NULL ? INSTPROXY_COMMAND_TYPE_SYNC : INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
 
 	plist_free(command);
 
@@ -677,7 +677,7 @@
 		plist_dict_set_item(command, "ClientOptions", plist_copy(client_options));
 	plist_dict_set_item(command, "PackagePath", plist_new_string(pkg_path));
 
-	res = instproxy_perform_command(client, command, INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
+	res = instproxy_perform_command(client, command, status_cb == NULL ? INSTPROXY_COMMAND_TYPE_SYNC : INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
 
 	plist_free(command);
 
@@ -694,7 +694,7 @@
 		plist_dict_set_item(command, "ClientOptions", plist_copy(client_options));
 	plist_dict_set_item(command, "ApplicationIdentifier", plist_new_string(appid));
 
-	res = instproxy_perform_command(client, command, INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
+	res = instproxy_perform_command(client, command, status_cb == NULL ? INSTPROXY_COMMAND_TYPE_SYNC : INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
 
 	plist_free(command);
 
@@ -727,7 +727,7 @@
 		plist_dict_set_item(command, "ClientOptions", plist_copy(client_options));
 	plist_dict_set_item(command, "ApplicationIdentifier", plist_new_string(appid));
 
-	res = instproxy_perform_command(client, command, INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
+	res = instproxy_perform_command(client, command, status_cb == NULL ? INSTPROXY_COMMAND_TYPE_SYNC : INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
 
 	plist_free(command);
 
@@ -744,7 +744,7 @@
 		plist_dict_set_item(command, "ClientOptions", plist_copy(client_options));
 	plist_dict_set_item(command, "ApplicationIdentifier", plist_new_string(appid));
 
-	res = instproxy_perform_command(client, command, INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
+	res = instproxy_perform_command(client, command, status_cb == NULL ? INSTPROXY_COMMAND_TYPE_SYNC : INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
 
 	plist_free(command);
 
@@ -761,7 +761,7 @@
 		plist_dict_set_item(command, "ClientOptions", plist_copy(client_options));
 	plist_dict_set_item(command, "ApplicationIdentifier", plist_new_string(appid));
 
-	res = instproxy_perform_command(client, command, INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
+	res = instproxy_perform_command(client, command, status_cb == NULL ? INSTPROXY_COMMAND_TYPE_SYNC : INSTPROXY_COMMAND_TYPE_ASYNC, status_cb, user_data);
 
 	plist_free(command);