bootctl: unify boot entry loading for "status" and "list"

We must be consistent in the two listings, so let's split out the loading code
and call it from both verb_status() and verb_list(). This effectively makes
verb_status() also call efi_loader_get_entries().

There is still some code duplicated, but that's hard to avoid. Error messages
are made identical for the same errors in various places.

Fixes #22580.

src/shared/bootspec.h

@@ -71,7 +71,7 @@ typedef struct BootConfig {
 
 BootEntry* boot_config_find_entry(BootConfig *config, const char *id);
 
-static inline BootEntry* boot_config_default_entry(BootConfig *config) {
+static inline const BootEntry* boot_config_default_entry(const BootConfig *config) {
         assert(config);
 
         if (config->default_entry < 0)