utf8: automatically determine length of string if SIZE_MAX is specified

Let's make utf8_to_utf16() and utf16_to_utf8() a bit nicer to use by
adding shortcuts for common cases.

This is particularly relevant for utf16_to_utf8() since the
multiplication with 2 is easy to forget.

src/boot/bootctl-set-efivar.c

@@ -34,7 +34,7 @@ static int parse_timeout(const char *arg1, char16_t **ret_timeout, size_t *ret_t
 
         xsprintf(utf8, USEC_FMT, MIN(timeout / USEC_PER_SEC, UINT32_MAX));
 
-        encoded = utf8_to_utf16(utf8, strlen(utf8));
+        encoded = utf8_to_utf16(utf8, SIZE_MAX);
         if (!encoded)
                 return log_oom();
 
@@ -69,7 +69,7 @@ static int parse_loader_entry_target_arg(const char *arg1, char16_t **ret_target
         } else if (arg1[0] == '@' && !streq(arg1, "@saved"))
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Unsupported special entry identifier: %s", arg1);
         else {
-                encoded = utf8_to_utf16(arg1, strlen(arg1));
+                encoded = utf8_to_utf16(arg1, SIZE_MAX);
                 if (!encoded)
                         return log_oom();
 

src/boot/efi/test-bcd.c

@@ -152,7 +152,7 @@ TEST(argv_bcds) {
 
                 char16_t *title = get_bcd_title(bcd, len);
                 if (title) {
-                        _cleanup_free_ char *title_utf8 = utf16_to_utf8(title, char16_strlen(title) * 2);
+                        _cleanup_free_ char *title_utf8 = utf16_to_utf8(title, SIZE_MAX);
                         log_info("%s: \"%s\"", saved_argv[i], title_utf8);
                 } else
                         log_info("%s: Bad BCD", saved_argv[i]);