MINOR: sample: add ARGC_OPT

Add ARGC_OPT enum to provide more context for upcoming sample parse errors
involving proxy "option" config directives.
diff --git a/src/sample.c b/src/sample.c
index 953e463..2e04c74 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -1310,6 +1310,7 @@
 		case ARGC_USRV:  where = "in use-server or balance expression in"; break;
 		case ARGC_HERR:  where = "in http-error directive in"; break;
 		case ARGC_OT:    where = "in ot-scope directive in"; break;
+		case ARGC_OPT:   where = "in option directive in"; break;
 		case ARGC_TCO:   where = "in tcp-request connection expression in"; break;
 		case ARGC_TSE:   where = "in tcp-request session expression in"; break;
 		case ARGC_TRQ:   where = "in tcp-request content expression in"; break;