Prevent lock-recursion in _stp_ctl_send

Prevent lock-recursion in _stp_ctl_send

In rare cases, we may hit a probe while the transport layer is holding a
spinlock, and that probe may call _stp_ctl_send which tries to grab the
same and deadlocks.  This is a bit easier to trigger on lockdep-enabled
kernels with the lock_acquired tracepoint.

This patch refactors the context->busy state management into get/put
context, and those areas which grab probe-sensitive locks now wrap
themselves with a context to stay comfortably free of probes.

Tangentially, the dyninst side abandons the busy flag, as it already had
a tls_context pointer to prevent direct recursion and a mutex for
exclusive access across all processes.

DEBUG_REENTRANCY is an unfortunate casualty, because we can't safely
call _stp_warn when the busy context may be from those held locks.