resolved: Avoid multiple SSL writes per DoT packet

In the DoT case, dns_stream_writev decomposed an iovec into multiple
dnstls_stream_write calls, which resulted in multiple SSL writes and multiple
TLS records. This can be checked from a network capture, e.g. using socat:
socat -v -x openssl-listen:853,reuseaddr,fork,cert=my.cert,key=my.key,verify=0 openssl:8.8.8.8:853

Instead, propagate the iovec as-is into the DoT handling code. For GnuTLS, the
library provides support for buffering ('corking') a record. OpenSSL has no
such facility, so we join the iovec into a single buffer then call SSL_write.

socat capture of `resolvectl -4 query --cache=no example.com` before the commit:

> 2022/01/30 13:35:52.194200  length=2 from=0 to=1
 00 28                                            .(
--
> 2022/01/30 13:35:52.194253  length=40 from=2 to=41
 1e b2 01 00 00 01 00 00 00 00 00 01 07 65 78 61  .............exa
 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01 00 00 29  mple.com.......)
 ff e4 00 00 00 00 00 00                          ........
--
< 2022/01/30 13:35:52.232798  length=58 from=0 to=57
 00 38 1e b2 81 80 00 01 00 01 00 00 00 01 07 65  .8.............e
 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01 c0  xample.com......
 0c 00 01 00 01 00 00 53 6f 00 04 5d b8 d8 22 00  .......So..]..".
 00 29 02 00 00 00 00 00 00 00                    .)........

socat capture of `resolvectl -4 query --cache=no example.com` after the commit:

> 2022/01/30 13:34:47.598099  length=42 from=504 to=545
 00 28 37 86 01 00 00 01 00 00 00 00 00 01 07 65  .(7............e
 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01 00  xample.com......
 00 29 ff e4 00 00 00 00 00 00                    .)........
--
< 2022/01/30 13:34:47.613203  length=58 from=756 to=813
 00 38 37 86 81 80 00 01 00 01 00 00 00 01 07 65  .87............e
 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01 c0  xample.com......
 0c 00 01 00 01 00 00 52 5e 00 04 5d b8 d8 22 00  .......R^..]..".
 00 29 02 00 00 00 00 00 00 00                    .)........

1 files changed, 2 insertions, 1 deletions

diff --git a/src/resolve/resolved-dnstls.h b/src/resolve/resolved-dnstls.h
index ed214dc6c4..70b27d8d77 100644
--- a/src/resolve/resolved-dnstls.h
+++ b/src/resolve/resolved-dnstls.h
@@ -5,6 +5,7 @@
 
 #include <stdbool.h>
 #include <stdint.h>
+#include <sys/uio.h>
 
 typedef struct DnsServer DnsServer;
 typedef struct DnsStream DnsStream;
@@ -27,7 +28,7 @@ int dnstls_stream_connect_tls(DnsStream *stream, DnsServer *server);
 void dnstls_stream_free(DnsStream *stream);
 int dnstls_stream_on_io(DnsStream *stream, uint32_t revents);
 int dnstls_stream_shutdown(DnsStream *stream, int error);
-ssize_t dnstls_stream_write(DnsStream *stream, const char *buf, size_t count);
+ssize_t dnstls_stream_writev(DnsStream *stream, const struct iovec *iov, size_t iovcnt);
 ssize_t dnstls_stream_read(DnsStream *stream, void *buf, size_t count);
 bool dnstls_stream_has_buffered_data(DnsStream *stream);