libbb: make index_in_substrings return -1 on ambiguous matches

function                                             old     new   delta
index_in_substrings                                   67      93     +26

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/udhcp/common.c b/networking/udhcp/common.c
index 6e8ec3e..adbcd77 100644
--- a/networking/udhcp/common.c
+++ b/networking/udhcp/common.c
@@ -435,8 +435,8 @@
 			}
 			break;
 //		case OPTION_BOOLEAN: {
-//			static const char noyes[] ALIGN1 = "no\0yes\0";
-//			buffer[0] = retval = index_in_strings(noyes, val);
+//			static const char no_yes[] ALIGN1 = "no\0yes\0";
+//			buffer[0] = retval = index_in_strings(no_yes, val);
 //			retval++; /* 0 - bad; 1: "no" 2: "yes" */
 //			break;
 //		}