bg_task_wakeup(): allow but loudly warn when called on a cancelled task!

Switched all logging from g_xxx() to s_xxx() routines since we are in the
library and g_xxx() should be used by application-level code only.
Regenerated Configure to allow proper BFD library detection for newest versions.
teq_post_rpc(): use linearily increasing timeouts to limit logging.

When timeouts occur in cross-thread RPCs, we need to give time to the remote
thread to purge its events, hence we can safely increase the timeout period
each time -- we will continue to log if there is indeed a bug, but we will limit
the amount of (verbose) logging done when we have a transient condition.
BG tasks: explictly record that a task has been put into user-induced sleep.
bg_task_cancel(): added debugging code to warn when cancelling a sleeping task.