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/resolv/res_query.cpp b/resolv/res_query.cpp
index f5da81f..0f7b54e 100644
--- a/resolv/res_query.cpp
+++ b/resolv/res_query.cpp
@@ -70,6 +70,8 @@
  * OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
+#define LOG_TAG "res_query"
+
 #include <arpa/inet.h>
 #include <arpa/nameser.h>
 #include <ctype.h>