libdwfl: Add dwfl_getthread_frames.

libdwfl: Add dwfl_getthread_frames.

dwfl_getthread_frames is a convenience function for when the user is only
interested in one specific thread id of a process. It can be implemented by
a simple wrapper function that removes an extra callback layer just to
filter on thread id. But it also provides an optimized path to getting
access to just one particular Dwfl_Thread of the Dwfl process by providing
and (optional) new callback for the state provider. The pid_thread_callbacks
now provide an (optional) pid_getthread that doesn't need to travers all
threads anymore. Which is implemented for the linux-pid-attach provider.

stack now uses this to implement a new '-1' option that shows just one
specific thread of a process.

Signed-off-by: Mark Wielaard <mjw@redhat.com>