Optionally re-introduce bb_info_msg()

Between Busybox 1.24.2 and 1.25.0 the bb_info_msg() function was
eliminated and calls to it changed to be bb_error_msg(). The downside of
this is that daemons now log all messages to syslog at the LOG_ERR level
which makes it hard to filter errors from informational messages.

This change optionally re-introduces bb_info_msg(), controlled by a new
option FEATURE_SYSLOG_INFO, restores all the calls to bb_info_msg() that
were removed (only in applets that set logmode to LOGMODE_SYSLOG or
LOGMODE_BOTH), and also changes informational messages in ifplugd and
ntpd.

The code size change of this is as follows (using 'defconfig' on x86_64
with gcc 7.3.0-27ubuntu1~18.04)

function                                             old     new   delta
bb_info_msg                                            -     182    +182
bb_vinfo_msg                                           -      27     +27
static.log7                                          194     198      +4
log8                                                 190     191      +1
log5                                                 190     191      +1
crondlog                                              45       -     -45
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 3/0 up/down: 215/-45)           Total: 170 bytes

If you don't care about everything being logged at LOG_ERR level
then when FEATURE_SYSLOG_INFO is disabled Busybox actually gets smaller:

function                                             old     new   delta
static.log7                                          194     200      +6
log8                                                 190     193      +3
log5                                                 190     193      +3
syslog_level                                           1       -      -1
bb_verror_msg                                        583     581      -2
crondlog                                              45       -     -45
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 3/1 up/down: 12/-48)            Total: -36 bytes

Signed-off-by: James Byrne <james.byrne@origamienergy.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/dnsd.c b/networking/dnsd.c
index 37a8030..f2c6bdd 100644
--- a/networking/dnsd.c
+++ b/networking/dnsd.c
@@ -133,7 +133,7 @@
 		}
 
 		if (OPT_verbose)
-			bb_error_msg("name:%s, ip:%s", token[0], token[1]);
+			bb_info_msg("name:%s, ip:%s", token[0], token[1]);
 
 		/* sizeof(*m) includes 1 byte for m->name[0] */
 		m = xzalloc(sizeof(*m) + strlen(token[0]) + 1);
@@ -438,7 +438,7 @@
 	answstr = table_lookup(conf_data, type, query_string);
 #if DEBUG
 	/* Shows lengths instead of dots, unusable for !DEBUG */
-	bb_error_msg("'%s'->'%s'", query_string, answstr);
+	bb_info_msg("'%s'->'%s'", query_string, answstr);
 #endif
 	outr_rlen = 4;
 	if (answstr && type == htons(REQ_PTR)) {
@@ -474,7 +474,7 @@
 	 * RCODE = 0 "success"
 	 */
 	if (OPT_verbose)
-		bb_error_msg("returning positive reply");
+		bb_info_msg("returning positive reply");
 	outr_flags = htons(0x8000 | 0x0400 | 0);
 	/* we have one answer */
 	head->nansw = htons(1);
@@ -539,7 +539,7 @@
 
 	{
 		char *p = xmalloc_sockaddr2dotted(&lsa->u.sa);
-		bb_error_msg("accepting UDP packets on %s", p);
+		bb_info_msg("accepting UDP packets on %s", p);
 		free(p);
 	}
 
@@ -557,7 +557,7 @@
 			continue;
 		}
 		if (OPT_verbose)
-			bb_error_msg("got UDP packet");
+			bb_info_msg("got UDP packet");
 		buf[r] = '\0'; /* paranoia */
 		r = process_packet(conf_data, conf_ttl, buf);
 		if (r <= 0)