Fix debugging programs statically linked against the thread library.

Fix debugging programs statically linked against the thread library.
* thread-db.c: Various comment fixes and additions.
Include "bfd.h", "symfile.h" and "objfiles.h".
(keep_thread_db): New variable.
(find_new_threads_callback): Remove prototype.
(thread_db_find_new_threads): New prototype.
(thread_db_push_target, thread_db_unpush_target): Remove
functions.
(deactivate_target): New function.
(thread_db_new_objfile): If OBJFILE == NULL, force deactivation of
target vector.  Activate target vector directly instead of calling
thread_db_push_target.  Set keep_thread_db if thread library is
detected in the main symbol file.  Only enable thread event
reporting if there actually is a child process.  Likewise for
detecting new threads, done by calling thread_db_find_new_threads
instead of iterating over the threads ourselves.
(thread_db_detach): Call deactivate_target instead of
thread_db_unpush_target.
(thread_db_wait): Bail out early if we're not debugging the
multi-threaded child process yet.
(thread_db_post_startup_inferior): New function.
(thread_db_mourn_inferior): Call deactivate_target instead of
thread_db_unpush_target.
(init_thread_db_ops): Add thread_db_post_startup_inferior to
thread_db_ops.