firstboot: auto-fill keymap/locale questions with data from firmware, if available (#42177)

Let's make the installer experience a tiny bit nicer, and prefill
everything with the data the firmware gives us.

src/basic/terminal-util.c

@@ -283,6 +283,7 @@ static void clear_by_backspace(size_t n) {
 
 int ask_string_full(
                 char **ret,
+                const char *prefill,
                 GetCompletionsCallback get_completions,
                 void *userdata,
                 const char *text, ...) {
@@ -296,8 +297,17 @@ int ask_string_full(
         _cleanup_free_ char *string = NULL;
         size_t n = 0;
 
-        if (get_completions) {
-                /* Figure out what string to preselect the query with */
+        if (prefill) {
+                /* Prefill query with explicit data if specified */
+
+                string = strdup(prefill);
+                if (!string)
+                        return -ENOMEM;
+
+                n = strlen(string);
+
+        } else if (get_completions) {
+                /* Otherwise, figure out what string to preselect the query with */
                 _cleanup_strv_free_ char **completions = NULL;
                 r = get_completions("", GET_COMPLETIONS_PRESELECT, &completions, userdata);
                 if (r < 0)

src/basic/terminal-util.h

@@ -95,8 +95,8 @@ typedef enum GetCompletionsFlags {
 } GetCompletionsFlags;
 
 typedef int (*GetCompletionsCallback)(const char *key, GetCompletionsFlags flags, char ***ret_list, void *userdata);
-int ask_string_full(char **ret, GetCompletionsCallback get_completions, void *userdata, const char *text, ...) _printf_(4, 5);
-#define ask_string(ret, text, ...) ask_string_full(ret, NULL, NULL, text, ##__VA_ARGS__)
+int ask_string_full(char **ret, const char *prefill, GetCompletionsCallback get_completions, void *userdata, const char *text, ...) _printf_(5, 6);
+#define ask_string(ret, text, ...) ask_string_full(ret, NULL, NULL, NULL, text, ##__VA_ARGS__)
 
 bool any_key_to_proceed(void);
 int show_menu(char **x, size_t n_columns, size_t column_width, unsigned ellipsize_percentage, const char *grey_prefix, bool with_numbers);