resolved: Avoid multiple SSL writes per DoT packet

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                    .)........