fs: add recursive subdirectory support to fs.watch

Currently fs.watch does not have an option to specify if a directory
should be recursively watched for events across all subdirectories.

Several file watcher APIs support this. FSEvents on OS X > 10.5 is
one example. libuv has added support for FSEvents, but fs.watch had
no way to specify that a recursive watch was required.

fs.watch now has an additional boolean option 'recursive'. When set
to true, and when supported, fs.watch will return notifications for
the entire directory tree hierarchy rooted at the specified path.
tls: reuse hostname from underlying net.Socket

When `tls.connect()` is called with `socket` option, it should try to
reuse hostname previously passed to `net.connect()` and only after that
fall back to `'localhost'`.

fix #6409
tls: do not default to 'localhost' servername
build: use zero overhead systemtap probes

Profiling suggested that on Linux sometimes over 10% of CPU time was
being spent inside the systemtap probe entry points in the binding
layer, even when the process was not actively being traced with the
`stap` tool.

That's why this commit makes it possible to use the *_ENABLED() macros
and bail out early when we're not being traced, reducing the overhead
of unused probes to (almost) zero.

Said macros were already being generated by `dtrace -h` but were not
usable because they rely on external definitions.  To remedy that, we
now generate the accompanying object files with `dtrace -G`.

This commit includes a change to libuv that has been landed upstream in
commit joyent/libuv@3c172ea.
test: fix tls-client-reject after ba7c9ce96