Log Message:
Fix lib/thread.c implementation for list handling and thread_cancellation:

Documented upstream in: https://bugzilla.quagga.net/show_bug.cgi?id=1011

The included patches will remedy thread.c shortcomings and add proper safeguards
to detect future thread handling errors.
fixes in the patches:
 - add a name to a thread for error messages
 - add a list pointer to the thread to track list membership
 - add fast check on prev/next pointer invariants
 - add check to detect inserting of a thread already in a list
 - add check to detect deletion of a thread not in a list
 - ignore cancellation requests for the currently running thread (fixes crash)
 - fix setting of prey/next pointers on adjecant elements when a head/tail element is
   deleted