sysupdate: parse PartitionFlags= as hexadecimal

sysupdate.d(5) documents PartitionFlags= as a hexadecimal integer, and
sysupdate patterns format partition flags with PRIx64. The parser used
safe_atou64(), which rejected values such as abcdef.

Reproducer: set PartitionFlags=abcdef in a partition target transfer
and run systemd-sysupdate list with debug logging.

Before: sysupdate logged:
Failed to parse partition flags, ignoring: abcdef

After: the transfer parses cleanly and list emits no partition-flags
parse warning.

Follow-up for: 43cc7a3ef4

src/sysupdate/sysupdate-transfer.c

@@ -471,7 +471,7 @@ static int config_parse_partition_flags(
 
         assert(rvalue);
 
-        r = safe_atou64(rvalue, &t->partition_flags);
+        r = safe_atoux64(rvalue, &t->partition_flags);
         if (r < 0) {
                 log_syntax(unit, LOG_WARNING, filename, line, r,
                            "Failed to parse partition flags, ignoring: %s", rvalue);