nss-resolve: initialize logging, log json errors

When the .so module is loaded, it gets a separate copy of stuff in src/basic,
including the log level variables. So any logging settings are unaffected by
the loading program calling log_parse_environment() or such. Let's also parse
the environment here so that we can have nice logging.

Initialization is done from each exported function, and pthread_once_t is used
to avoid duplicate initialization. I didn't merge PROTECT_ERRNO into
NSS_ENTRYPOINT_BEGIN because UNPROTECT_ERRNO is called in a bunch of places
and it would feel strange to have PROTECT_ERRNO hidden, but not UNPROTECT_ERRNO.

The most interesting stuff in this module is the varlink messages, and any
potential errors in json. So let's enable json logging when debug messages are
enabled.

With those changes, figuring out the issue in
https://github.com/systemd/systemd/pull/17823 is trivial:

$ LD_LIBRARY_PATH=build/ SYSTEMD_LOG_COLOR=1 SYSTEMD_LOG_LOCATION=1 SYSTEMD_LOG_LEVEL=debug getent hosts mirrors.fedoraproject.org
src/shared/varlink.c:237: n/a: varlink: setting state idle-client
src/shared/varlink.c:1240: n/a: Sending message: {"method":"io.systemd.Resolve.ResolveHostname","parameters":{"name":"mirrors.fedoraproject.org","family":10}}
src/shared/varlink.c:240: n/a: varlink: changing state idle-client → calling
src/shared/varlink.c:588: n/a: New incoming message: {"parameters":{"addresses":[{"ifindex":0,"family":10,"address":[42,5,208,20,0,16,120,3,247,116,77,124,226,119,164,87]},{"ifindex":0,"family":10,"address":[42,5,208,28,12,106,204,3,38,58,132,9,185,97,126,2]},{"ifindex":0,"family":10,"address":[38,32,0,82,0,3,0,1,222,173,190,239,202,254,254,215]},{"ifindex":0,"family":10,"address":[38,5,188,128,48,16,6,0,222,173,190,239,202,254,254,217]},{"ifindex":0,"family":10,"address":[38,4,21,128,254,0,0,0,222,173,190,239,202,254,254,209]},{"ifindex":0,"family":10,"address":[38,32,0,82,0,3,0,1,222,173,190,239,202,254,254,214]},{"ifindex":0,"family":10,"address":[38,16,0,40,48,144,48,1,222,173,190,239,202,254,254,211]},{"ifindex":0,"family":10,"address":[32,1,65,120,0,2,18,105,0,0,0,0,0,0,254,210]}],"name":"wildcard.fedoraproject.org","flags":1}}
src/shared/varlink.c:240: n/a: varlink: changing state calling → called
src/shared/varlink.c:240: n/a: varlink: changing state called → idle-client
src/nss-resolve/nss-resolve.c:84: (string):1:40: JSON field 'ifindex' is out of bounds for an interface index.