Set exit status to 1 if strace has not traced any processes

* strace.c (main): Initialize exit_code with 1 when no processes
has been attached.
* strace.1: Document it.
* NEWS: Mention this change.

NEWS

@@ -1,6 +1,10 @@
 Noteworthy changes in release ?.?? (????-??-??)
 ===============================================
 
+* Changes in behavior
+  * When using -p option without a command and no processes has been attached,
+    strace exits with exit status 1.
+
 Noteworthy changes in release 4.13 (2016-07-26)
 ===============================================
 

strace.1

@@ -601,10 +601,13 @@ unless
 is used.
 .LP
 When using
-.BR \-p ,
+.B \-p
+without a
+.IR command ,
 the exit status of
 .B strace
-is zero unless there was an unexpected error in doing the tracing.
+is zero unless no processes has been attached or there was an unexpected error
+in doing the tracing.
 .SH "SETUID INSTALLATION"
 If
 .B strace

strace.c

@@ -138,7 +138,7 @@ static bool skip_one_b_execve = 0;
 /* Are we "strace PROG" and need to hide everything until execve? */
 bool hide_log_until_execve = 0;
 
-static int exit_code = 0;
+static int exit_code;
 static int strace_child = 0;
 static int strace_tracer_pid = 0;
 
@@ -2387,6 +2387,8 @@ main(int argc, char *argv[])
 {
 	init(argc, argv);
 
+	exit_code = !nprocs;
+
 	while (trace())
 		;