nss-resolve: fix parsing of io.systemd.Resolve.ResolveAddress reply

nss-resolve: fix parsing of io.systemd.Resolve.ResolveAddress reply

Since the switch to varlink in 0c73f4f075a2d23f7cabe708b589f19f4bbbec37, the
code wasn't functional. The JSON_VARIANT_UNSIGNED/JSON_VARIANT_STRING mismatch
meant that we'd reject any reply. Once past that, the code would use
unitialized 'c' and 'n' variables, so it's lucky we never got that far ;)

With -Wmaybe-unitialized, gcc would warn.

I think that declaring the huge list of local variables with very short names
at the top of the function was making it harder to understand what is going on
in the function. So let's rename the variables a bit, and initialize them upon
declaration if possible.

$ build/test-nss-hosts resolve 1.1.1.1 1.0.0.1 10.38.5.41
======== resolve ========
_nss_resolve_gethostbyaddr2_r("1.1.1.1") → status=NSS_STATUS_SUCCESS
                   errno=999/--- h_errno=0/Resolver Error 0 (no error) ttl=0
        "one.one.one.one"
        AF_INET 1.1.1.1

_nss_resolve_gethostbyaddr_r("1.1.1.1") → status=NSS_STATUS_SUCCESS
                   errno=999/--- h_errno=0/Resolver Error 0 (no error)
        "one.one.one.one"
        AF_INET 1.1.1.1

_nss_resolve_gethostbyaddr2_r("1.0.0.1") → status=NSS_STATUS_SUCCESS
                   errno=999/--- h_errno=0/Resolver Error 0 (no error) ttl=0
        "one.one.one.one"
        AF_INET 1.0.0.1

_nss_resolve_gethostbyaddr_r("1.0.0.1") → status=NSS_STATUS_SUCCESS
                   errno=999/--- h_errno=0/Resolver Error 0 (no error)
        "one.one.one.one"
        AF_INET 1.0.0.1

_nss_resolve_gethostbyaddr2_r("10.38.5.41") → status=NSS_STATUS_SUCCESS
                   errno=999/--- h_errno=0/Resolver Error 0 (no error) ttl=0
        "squid.redhat.com"
        alias "squid.corp.redhat.com"
        alias "squid2.corp.redhat.com"
        alias "squid3.corp.redhat.com"
        alias "squid4.corp.redhat.com"
        alias "squid5.corp.redhat.com"
        AF_INET 10.38.5.41

_nss_resolve_gethostbyaddr_r("10.38.5.41") → status=NSS_STATUS_SUCCESS
                   errno=999/--- h_errno=0/Resolver Error 0 (no error)
        "squid.redhat.com"
        alias "squid.corp.redhat.com"
        alias "squid2.corp.redhat.com"
        alias "squid3.corp.redhat.com"
        alias "squid4.corp.redhat.com"
        alias "squid5.corp.redhat.com"
        AF_INET 10.38.5.41

(I have 10.38.5.41 squid.redhat.com squid.corp.redhat.com squid2.corp.redhat.com squid3.corp.redhat.com squid4.corp.redhat.com squid5.corp.redhat.com
in /etc/hosts for testing.)

(cherry picked from commit 77fac974fe396dbe4fb679b748bfa89db1136e0c)