cryptsetup: modify keyfile search logic to use read_file_full() too

Let's move the 3rd way how cryptsetup acquires key files to
read_file_full() too.

Since load_key_file()'s raison d'etre now is just the search path logic,
let's rename the function to find_key_file().

src/cryptsetup/cryptsetup-keyfile.h

@@ -4,10 +4,9 @@
 #include <inttypes.h>
 #include <sys/types.h>
 
-int load_key_file(
+int find_key_file(
                 const char *key_file,
                 char **search_path,
-                size_t key_file_size,
-                uint64_t key_file_offset,
+                const char *bindname,
                 void **ret_key,
                 size_t *ret_key_size);