bootctl: synchronize bootctl code with sd-boot code

This patch adds a line with "#console-mode keep" as a documentation for
the console mode feature and duplicates console-mode parsing to bootctl.

src/boot/bootctl.c

@@ -826,6 +826,7 @@ static int install_loader_config(const char *esp_path) {
         }
 
         fprintf(f, "#timeout 3\n");
+        fprintf(f, "#console-mode keep\n");
         fprintf(f, "default %s-*\n", sd_id128_to_string(machine_id, machine_string));
 
         r = fflush_sync_and_check(f);

src/shared/bootspec.c

@@ -194,6 +194,8 @@ int boot_loader_read_conf(const char *path, BootConfig *config) {
                         r = free_and_strdup(&config->timeout, p);
                 else if (streq(buf, "editor"))
                         r = free_and_strdup(&config->editor, p);
+                else if (streq(buf, "console-mode"))
+                        r = free_and_strdup(&config->console_mode, p);
                 else {
                         log_notice("%s:%u: Unknown line \"%s\"", path, line, buf);
                         continue;

src/shared/bootspec.h

@@ -40,6 +40,7 @@ typedef struct BootConfig {
         char *default_pattern;
         char *timeout;
         char *editor;
+        char *console_mode;
 
         char *entry_oneshot;
         char *entry_default;