resolv: small improvements for debug logging

 - use __func__ instead of the hand-written and often incorrect names
 - fix a few cosmetic issues, such as those ugly ";;" prefixes
 - add some logging around explore_fqdn and explore_numeric
 - change the verbosity of a few messages
 - replace compile-time DBG macro with verbosity checks in DnsProxyListener
 - delete various unused functions in res_debug
 - skip slow res_pquery() if log level is less than VERBOSE

TODO: convert the remaining ALOGx() lines to LOG(x). Leaving this for a
future patch.

Test: as follows:
    adb shell setprop persist.sys.nw_dns_resolver_log DEBUG
    atest reolv_integration_test

Change-Id: I7153f0c75d39d513fa006e0c1e5bf649d47ba154
diff --git a/res_init.cpp b/res_init.cpp
index a02e7f0..0ec645e 100644
--- a/res_init.cpp
+++ b/res_init.cpp
@@ -69,6 +69,9 @@
  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
  * OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
+
+#define LOG_TAG "res_init"
+
 #include <sys/param.h>
 #include <sys/socket.h>
 #include <sys/time.h>
@@ -187,9 +190,8 @@
             dots--;
         }
         *pp = NULL;
-        LOG(DEBUG) << ";; res_init()... default dnsrch list:";
-        for (pp = statp->dnsrch; *pp; pp++) LOG(DEBUG) << ";;\t" << *pp;
-        LOG(DEBUG) << ";;\t..END..";
+        LOG(DEBUG) << "res_init(): dnsrch list:";
+        for (pp = statp->dnsrch; *pp; pp++) LOG(DEBUG) << "\t" << *pp;
     }
 
     if ((cp = getenv("RES_OPTIONS")) != NULL) res_setoptions(statp, cp, "env");