Use dynamically-sized selected[] array for -P PATH

While at it, added a small optimization of not remembering
the path twice if it happens to be the same.

   text	   data	    bss	    dec	    hex	filename
 245111	    680	  10860	 256651	  3ea8b	strace_old
 245075	    680	   9804	 255559	  3e647	strace

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/strace.c b/strace.c
index 319dff2..9ec354d 100644
--- a/strace.c
+++ b/strace.c
@@ -1649,9 +1649,7 @@
 			break;
 		case 'P':
 			tracing_paths = 1;
-			if (pathtrace_select(optarg)) {
-				error_msg_and_die("Failed to select path '%s'", optarg);
-			}
+			pathtrace_select(optarg);
 			break;
 		case 's':
 			i = string_to_uint(optarg);