dtrace: USDT implementation (Phase 1)

dtrace: USDT implementation (Phase 1)

This rather large patch provides the implementation for USDT support at the
DTrace kernel core level, and the fasttrap provider level.  It ensures that
executables can register their embedded providers (with USDT probes), that
the probes are visible to the dtrace userspace utility, and that probes are
properly removed upon executable completion, execve() invocation, or any
unexpected executable termination.

The following parts are provided by this patch:
- meta-provider support (dtrace_ptofapi)
- helper ioctl interface (dtrace_dev)
- DIF validation for helper objects (dtrace_dif)
- DOF processing for helpers for provider and probe definitions (dtrace_dof)
- fasttrap meta-provider for USDT only (fasttrap*)

The dtrace_helper.c file was removed because this code belongs in dtrace_dof.c
instead.

Minimal changes were made to the core kernel in exec.c, sched.h, exit.c, and
fork.c to add support for process-specific helpers (and those encapsulate
providers and probes).

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>