i#37: Add controlled ptrace injection on Linux

This adds an "-attach <pid>" option to drrun_exe on Linux and a
dr_inject_use_ptrace() routine to drinjectlib.  Full targeted injection
is not tested or supported yet; this is just the basis of the ptrace
implementation.

Review URL: https://codereview.appspot.com/6874052

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1877 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6