resolved: Make event flags logic robust for DoT

resolved: Make event flags logic robust for DoT

Since when handling a DNS over TLS stream, the TLS library can override the
requested events through dnstls_events for handshake/shutdown purposes,
obtaining the event flags through sd_event_source_get_io_events and checking
for EPOLLIN or EPOLLOUT does not really tell us whether we want to read/write
a packet. Instead, it could just be OpenSSL/GnuTLS doing something else.

To make the logic more robust (and simpler), save the flags that tell us
whether we want to read/write a packet, and check them instead of the IO flags.

(& use uint32_t for the flags like in sd_event_source_set_io_events prototype)