net: emit dns 'lookup' event before connect

net.connect() and net.createConnection() now emit a 'lookup' event
after resolving the hostname but before connecting.

Fixes #5418.
http: don't escape request path, reject bad chars

Commit 38149bb changes http.get() and http.request() to escape unsafe
characters. However, that creates an incompatibility with v0.10 that
is difficult to work around: if you escape the path manually, then in
v0.11 it gets escaped twice. Change lib/http.js so it no longer tries
to fix up bad request paths, simply reject them with an exception.

The actual check is rather basic right now. The full check for illegal
characters is difficult to implement efficiently because it requires a
few characters of lookahead. That's why it currently only checks for
spaces because those are guaranteed to create an invalid request.

Fixes #5474.
module: use path.sep instead of custom solution

Instead of using a custom solution for path delimiters we can use
path.sep in Module._initPaths
dtrace: enable uv's probes if enabled
http: don't slice unless necessary

The incoming Buffer was being sliced just to possibly not be used.
Instead place the .slice inside the conditional to not be created unless
necessary.