linux attach: support attaching to processes with non-pthreads threads

_From [rnk@google.com](https://code.google.com/u/rnk@google.com/) on May 03, 2012 13:12:32_

Splitting from issue #38 and issue #722 .

Our current approach for discovering threads in the current process is to look in /proc/self/task, which contains thread ids for all threads in the current thread group. If a process uses vfork or clone without CLONE_THREAD, then we'll miss them. Other threads will be executing in the current address space, and bad things are likely to happen.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=764_

Start by tracing the Linux attach logic that discovers threads through /proc/self/task and read the related context from issues #38 and #722. Done means processes using vfork or clone without CLONE_THREAD are discovered and handled safely; no specific source file or test is named in the issue.

Written by the indexing model from the issue text.