Fix the case where we try to detach unattached processes

Fix the case where we try to detach unattached processes

Before this change:
$ strace -D -p1
strace: -D and -p are mutually exclusive options
Process 1 detached  <==== WRONG! (and we try to SIGSTOP it!!!)

* defs.h: Change the meaning of TCB_ATTACHED: now it means "this tracee
is attached to us". Add TCB_STRACE_CHILD: "this tracee is our child".
* strace.c (kill_save_errno): Move up. No code changes.
(process_opt_p_list): Don't set TCB_ATTACHED on new tcb.
(startup_attach): Change how we work with TCB_ATTACHED.
Set TCB_STRACE_CHILD on -D.
(startup_child): Use kill_save_errno instead of kill.
Set TCB_ATTACHED and TCB_STRACE_CHILD on attached strace child.
If we are in -D case, don't set TCB_ATTACHED (we aren't attached yet).
(detach): do not do PTRACE_DETACH if TCB_ATTACHED is not set.
(cleanup): Check TCB_STRACE_CHILD instead of TCB_ATTACHED.
(trace): Likewise.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>