Limit C linkage to public symbols

Switched all internal functions to C++ linkage. This makes it harder to
accidentally call a function with a mismatched signature, and paves the
way to using C++ data structures internally.

This is the current list of symbols expoted by libnetd_resolv.so:
  _resolv_delete_cache_for_net
  _resolv_flush_cache_for_net
  _resolv_set_nameservers_for_net
  android_getaddrinfofornet
  android_getaddrinfofornetcontext
  android_gethostbyaddrfornet
  android_gethostbyaddrfornetcontext
  android_gethostbynamefornet
  android_gethostbynamefornetcontext
  android_net_res_stats_aggregate
  android_net_res_stats_get_info_for_net
  android_net_res_stats_get_usable_servers

A mass-renaming pass would improve consistency. Perhaps we could use
the prefix "netd_resolv_", or just "resolv_" for brevity. Once we begin
shipping netd binaries using this interface, we might have to live with
it for some time.

Test: atest netd_integration_test (after flashing with new netd binary)
Change-Id: I52f32add73fd908ad4a715ef8f8aff1f8d9733d0
diff --git a/res_mkquery.cpp b/res_mkquery.cpp
index bda6ad9..82c42d5 100644
--- a/res_mkquery.cpp
+++ b/res_mkquery.cpp
@@ -85,30 +85,26 @@
 #define DEBUG
 #endif
 
-#define UNUSED(a) (void) &a
-
 extern const char* _res_opcodes[];
 
 /*
  * Form all types of queries.
  * Returns the size of the result or -1.
  */
-int res_nmkquery(res_state statp, int op, /* opcode of query */
-                 const char* dname,       /* domain name */
-                 int cl, int type,        /* class and type of query */
-                 const u_char* data,      /* resource record data */
-                 int datalen,             /* length of data */
-                 const u_char* newrr_in,  /* new rr for modify or append */
-                 u_char* buf,             /* buffer to put query */
-                 int buflen)              /* size of buffer */
+int res_nmkquery(res_state statp, int op,    /* opcode of query */
+                 const char* dname,          /* domain name */
+                 int cl, int type,           /* class and type of query */
+                 const u_char* data,         /* resource record data */
+                 int datalen,                /* length of data */
+                 const u_char* /*newrr_in*/, /* new rr for modify or append */
+                 u_char* buf,                /* buffer to put query */
+                 int buflen)                 /* size of buffer */
 {
     HEADER* hp;
     u_char *cp, *ep;
     int n;
     u_char *dnptrs[20], **dpp, **lastdnptr;
 
-    UNUSED(newrr_in);
-
 #ifdef DEBUG
     if (statp->options & RES_DEBUG)
         printf(";; res_nmkquery(%s, %s, %s, %s)\n", _res_opcodes[op], dname, p_class(cl),