Bail out of "attach" if a thread cannot be traced

Bail out of "attach" if a thread cannot be traced

On Linux, threads are treated much like separate processes by the
kernel.  In particular, it's possible to ptrace just a single thread.
If gdb tries to attach to a multi-threaded inferior, where a non-main
thread is already being traced (e.g., by strace), then gdb will get
into an infinite loop attempting to attach.

This patch fixes this problem by having the attach fail if ptrace
fails to attach to any thread of the inferior.