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/basic/efivars.c

@@ -265,7 +265,7 @@ finish:
 int efi_set_variable_string(const char *variable, const char *value) {
         _cleanup_free_ char16_t *u16 = NULL;
 
-        u16 = utf8_to_utf16(value, strlen(value));
+        u16 = utf8_to_utf16(value, SIZE_MAX);
         if (!u16)
                 return -ENOMEM;