gcoap_dns: various fixes [backport 2022.07]

[miri64]

Backport of #18367

Contribution description

While doing some more experiments with the current version of gcoap_dns, I noticed some bugs, specifically, when using block-wise transfer. This PR fixes those and also replaces some left-over printfs with DEBUG.

Testing procedure

Similar to the testing procedures in #16705:

$ sudo dist/tools/tapsetup/tapsetup
$ sudo ip addr add 2001:db8::1 dev tapbr0
$ sudo ip route add 2001:db8::/64 via "<native link-local>" dev tapbr0
$ pip install --upgrade git+https://github.com/miri64/aiodnsprox.git@exp  # blockwise does not work with upstream aiodnsprox yet
$ cat << EOF > config.yaml
dtls_credentials:
  client_identity: Client_identity
  psk: secretPSK
mock_dns_upstream:
  IN:
    A: 192.0.2.7
    AAAA: 2001:db8::7
$ aiodns-proxy -C config.yaml -c 2001:db8::1

RIOT_CONFIG_KCONFIG_USEMODULE_GCOAP_DNS=y \
   RIOT_CONFIG_KCONFIG_USEMODULE_GCOAP=y \
   RIOT_CONFIG_GCOAP_DNS_PDU_BUF_SIZE=100 \
   RIOT_CONFIG_GCOAP_DNS_BLOCK_SIZE=16 \
   RIOT_CONFIG_GCOAP_RESEND_BUFS_MAX=2 \
   make -C tests/gcoap_dns/ flash -j term

> ifconfig 7 add 2001:db8::2
ifconfig 7 add 2001:db8::2
success: added 2001:db8::2/64 to interface 7
> nib route add 7 default fe80::dc1a:a8ff:fe09:45b3
nib route add 7 default fe80::dc1a:a8ff:fe09:45b3
> uri coap://[2001:db8::1]/dns
uri coap://[2001:db8::1]/dns
Successfully added URI coap://[2001:db8::1]/dns
> query example.org inet6
query example.org inet6
Hostname example.org resolves to 2001:db8::7 (IPv6)

Wiershark also shows blockwise transfer for both request and response:

Without this fix query example.org inet6 will just fail with Unable to resolve query: Invalid argument (stemming from the wrongly reported -EINVAL).

Issues/PRs references

Follow-up on #16705

[miri64]

Rebased to current master.