src: notify V8 profiler when we're idle

Inform V8's CPU profiler when we're idle.  The profiler is
sampling-based but not all samples are created equal; mark the wall
clock time spent in epoll_wait() and friends so profiling tools can
filter it out.  The samples still end up in v8.log but with state=IDLE
rather than state=EXTERNAL.
src: only start idle notifier when profiling

The previous commit adds a notifier that tells the V8 profiler when
node.js is idle, i.e. when it's about to start sleeping in the
platform's equivalent of epoll_wait().

This commit adds a heuristic that only starts the notifier when the
V8 profiler is started from the command line.
src: add JS start/stop methods for idle notifier

The previous commit changes the profiler idle notifier so that it only
gets started when a --prof or --prof_lazy argument is specified on the
command line.

This commit adds two internal methods to the process object that allows
one to start and stop the idle notifier programmatically.
src: fix up after botched merge conflict

Mea culpa, I didn't properly resolve a merge conflict in the last two
commits.  The resulting segmentation fault only happened on Linux and
only sometimes.

Fixes #6306.
debugger: count space for line numbers correctly
v8: ugprade to 3.20.17.14
zlib: fix write request reference counting

Keep track of the reference count, don't make the wrapper object weak
when there are pending write requests.  Fixes a regression from c79d516.
dtrace, mdb_v8: support more string, frame types

This change makes several improvements to the ustack helper and MDB
support:

- ustack helper and MDB: add support for two-byte strings
  (necessary to print many filenames in stacktraces in 0.10 and later).
- ustack helper: fix position numbers, which were off by a factor of two
- ustack helper: fix frames with undefined Scripts (e.g., "RegExp")
- ustack helper: add stub frames
- MDB: add support for sliced strings
- MDB: sync up with changes from the illumos version of the module

Fixes #6309
Closes #6318
mdb_v8: remove useless check