libc/resolv: Export __res_conf_name

Add a new global variable, __res_conf_name, which is initialized to
_PATH_RESCONF and used in its place by res_init().  This allows test
programs (and applications) to select a different configuration file
to read instead of /etc/resolv.conf.

PR:		220610

lib/libc/resolv/Symbol.map

@@ -105,3 +105,7 @@ FBSD_1.0 {
 FBSD_1.4 {
 	__res_nrandomid;
 };
+
+FBSD_1.9 {
+	__res_conf_name;
+};

lib/libc/resolv/res_init.c

@@ -116,6 +116,8 @@ static int res_setsortlist(res_state, int, const char *, const char *);
 	(strncmp((str), (word), (end) - (str)) == 0 &&			\
 	    (word)[(end) - (str)] == '\0')
 
+const char *__res_conf_name = _PATH_RESCONF;
+
 /*
  * Resolver state default settings.
  */
@@ -214,7 +216,8 @@ __res_vinit(res_state statp, int preinit)
 	res_setservers(statp, u, nitems(u));
 
 	/* read the configuration file */
-	res_readconf(statp, _PATH_RESCONF, maxns);
+	if (__res_conf_name != NULL)
+		res_readconf(statp, __res_conf_name, maxns);
 
 	/* Allow user to override the local domain definition */
 	if ((cp = secure_getenv("LOCALDOMAIN")) != NULL) {