kernel-image: minor refactoring to inspect_kernel()

Let's add make three arguments optional, by splitting inspect_kernel()
from inspect_kernel_full().

Let's also downgrade logging to debug, so that this becomes more
library-like. Let's log on the call-site instead.

src/bootctl/bootctl-uki.c

@@ -5,14 +5,15 @@
 #include "alloc-util.h"
 #include "bootctl-uki.h"
 #include "kernel-image.h"
+#include "log.h"
 
 int verb_kernel_identify(int argc, char *argv[], uintptr_t _data, void *userdata) {
         KernelImageType t;
         int r;
 
-        r = inspect_kernel(AT_FDCWD, argv[1], &t, NULL, NULL, NULL);
+        r = inspect_kernel(AT_FDCWD, argv[1], &t);
         if (r < 0)
-                return r;
+                return log_error_errno(r, "Failed to inspect '%s': %m", argv[1]);
 
         puts(kernel_image_type_to_string(t));
         return 0;
@@ -23,9 +24,9 @@ int verb_kernel_inspect(int argc, char *argv[], uintptr_t _data, void *userdata)
         KernelImageType t;
         int r;
 
-        r = inspect_kernel(AT_FDCWD, argv[1], &t, &cmdline, &uname, &pname);
+        r = inspect_kernel_full(AT_FDCWD, argv[1], &t, &cmdline, &uname, &pname);
         if (r < 0)
-                return r;
+                return log_error_errno(r, "Failed to inspect '%s': %m", argv[1]);
 
         printf("Kernel Type: %s\n", kernel_image_type_to_string(t));
         if (cmdline)