libbb: new option FEATURE_ETC_SERVICES: if off, /etc/services reads often avoided

In practice, "wget http://host.com/" always uses port 80.
People explicitly set non-standard ports via options or parameters
("telnet 1.2.3.4 567" or "telnet 1.2.3.4 ftp") instead of modifying
/etc/services.

function                                             old     new   delta
telnet_main                                         1466    1464      -2
rdate_main                                           215     198     -17
fakeidentd_main                                      269     252     -17
parse_url                                            459     392     -67
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-103)           Total: -103 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/isrv_identd.c b/networking/isrv_identd.c
index 133d62a..0c33dde 100644
--- a/networking/isrv_identd.c
+++ b/networking/isrv_identd.c
@@ -159,7 +159,7 @@
 	fd = 0;
 	if (!(opt & OPT_inetdwait)) {
 		fd = create_and_bind_stream_or_die(bind_address,
-				bb_lookup_port("identd", "tcp", 113));
+				bb_lookup_std_port("identd", "tcp", 113));
 		xlisten(fd, 5);
 	}