Make res_init() work again.

Change 75830fb836621ebbcf68155e466983eb231f9ca1 to fix _nres
initialization to be thread safe accidentally introduced a behavior
change whereby res_init() became a no-op. It also failed to remove all
direct accesses to _nres.

Move the file over to C++ so we can let RAII ensure we're always holding
a lock while using the global state, make all callers access the global
state via this class, and restore the previous behavior of res_init().

Test: atest DnsResolverTest
Bug: 166235340
Change-Id: Ib390a7eac063bc0ff5eeba755e8c74ef1383004e
diff --git a/libc/dns/include/resolv_private.h b/libc/dns/include/resolv_private.h
index 77b03bf..3054555 100644
--- a/libc/dns/include/resolv_private.h
+++ b/libc/dns/include/resolv_private.h
@@ -51,8 +51,7 @@
  *	Id: resolv.h,v 1.7.2.11.4.2 2004/06/25 00:41:05 marka Exp
  */
 
-#ifndef _RESOLV_PRIVATE_H_
-#define	_RESOLV_PRIVATE_H_
+#pragma once
 
 #include <sys/cdefs.h>
 
@@ -63,6 +62,8 @@
 #include <net/if.h>
 #include <time.h>
 
+__BEGIN_DECLS
+
 // Linux defines MAXHOSTNAMELEN as 64, while the domain name limit in
 // RFC 1034 and RFC 1035 is 255 octets.
 #ifdef MAXHOSTNAMELEN
@@ -293,8 +294,6 @@
 /*			0x00010000	*/
 
 /* Things involving an internal (static) resolver context. */
-__BEGIN_DECLS
-
 __LIBC_HIDDEN__ extern struct __res_state *__res_get_state(void);
 __LIBC_HIDDEN__ extern void __res_put_state(struct __res_state *);
 
@@ -307,8 +306,6 @@
 #define _res (*__res_state())
 #endif
 
-__END_DECLS
-
 #ifndef __BIND_NOSTATIC
 #define fp_nquery		__fp_nquery
 #define fp_query		__fp_query
@@ -319,7 +316,6 @@
 #define res_isourserver		__res_isourserver
 #define	res_querydomain		__res_querydomain
 #define res_send		__res_send
-#define res_sendsigned		__res_sendsigned
 
 #ifdef BIND_RES_POSIX3
 #define	dn_expand	__dn_expand
@@ -329,7 +325,6 @@
 #define	res_mkquery	__res_mkquery
 #endif
 
-__BEGIN_DECLS
 void		fp_nquery(const u_char *, int, FILE *);
 void		fp_query(const u_char *, FILE *);
 const char *	hostalias(const char *);
@@ -343,8 +338,6 @@
 int		res_querydomain(const char *, const char *, int, int, u_char *, int);
 int		res_search(const char *, int, int, u_char *, int);
 int		res_send(const u_char *, int, u_char *, int);
-int		res_sendsigned(const u_char *, int, ns_tsig_key *, u_char *, int);
-__END_DECLS
 #endif
 
 #if !defined(SHARED_LIBBIND) || defined(LIB)
@@ -424,7 +417,6 @@
 #define	res_send_setrhook	__res_send_setrhook
 #define	res_servicename		__res_servicename
 #define	res_servicenumber	__res_servicenumber
-__BEGIN_DECLS
 int		res_hnok(const char *);
 int		res_ownok(const char *);
 int		res_mailok(const char *);
@@ -532,5 +524,3 @@
 int ns_name_labels(ns_nname_ct, size_t);
 
 __END_DECLS
-
-#endif /* !_RESOLV_PRIVATE_H_ */