doc: crypto: document SPKAC additions

Document the SPKAC functionality that was added in commit 7f66e44.
Revert "crypto: add SPKAC support"

This reverts commit 7f66e44dc1e90e7abda2a9ed02d7e8163e1f6358.
Revert "doc: crypto: document SPKAC additions"

This reverts commit aa94450b31a28d6251df3b88fdd652ddd1e67a69.
doc: http: add cross-links for easier clicking

Make it a little easier to navigate the http module documentation by
turning class names and methods into links to the appropriate section.
doc: http: reword IncomingMessage 'close' event

The bit that says "before response.end() was called or able to flush"
doesn't apply to incoming streams.

Fixes #6359.
crypto: add SPKAC support

Implements new class 'Certificate' within crypto object for working
with SPKAC's (signed public key & challenge) natively.
doc: crypto: document SPKAC additions

Document the SPKAC functionality that was added in commit 7bf46ba.
http_parser: expose pause/resume method for parser
http: provide backpressure for pipeline flood

If a client sends a lot more pipelined requests than we can handle, then
we need to provide backpressure so that the client knows to back off.
Do this by pausing both the stream and the parser itself when the
responses are not being read by the downstream client.

Fix GH-6214
debugger: make busy loops SIGUSR1-interruptible

Commit 30e5366b ("core: Use a uv_signal for debug listener") changed
SIGUSR1 handling from a signal handler to libuv's uv_signal_*()
functionality to fix a race condition (and possible hang) in the
signal handler.

While a good change in itself, it made it impossible to interrupt
long running scripts.  When a script is stuck in a busy loop, control
never returns to the event loop, which in turn means the signal
callback - and therefore the debugger - is never invoked.

This commit changes SIGUSR1 handling back to a normal signal handler
but one that treads _very_ carefully.
debugger: fix SIGUSR1 bootstrap race condition

Before this commit, the SIGUSR1 signal handler wasn't installed until
late in the bootstrapping process and we were prone to miss signals
sent by other processes.

This commit installs an early-boot signal handler that merely records
the fact that we received a signal.  Once the debugger infrastructure
is in place, the signal is re-raised, kickstarting the debugger.

Among other things, this means that simple/test-debugger-client is
now _much_ less likely to fail.
dns: rename domain to hostname

A follow-up commit will save the domain name on the request object but
we can't call that property 'domain' because that gets intercepted by
src/node.cc and lib/domain.js to implement the node.js feature of the
same name.

To avoid confusion, rename all variables called 'domain' to 'hostname'.
dns: set hostname property on error object

Make debugging and logging easier: when a DNS lookup for a hostname
fails, set the hostname as a property on the error object.

Fixes #5393.
cpplint: disallow comma-first in C++
cpplint: disallow if one-liners
doc: child_process corrections and cleanups

- Make explicit that .disconnected is set before the disconnect event,
  and it is not allowed to send messages after calling .disconnect(),
  even while waiting for a delayed disconect event.
- Remove obsolete claim that explicit exit is required
- Describe silent: in the options for fork()
- Describe .connected as the property it is, not just as an aside in
  the disconnect() method
doc: cluster documentation cleanup and corrections

- fixed some incomprehensible wording ("event assigned to..."?)
- removed undocumented and unnecessary process properties from example
- corrected the docs on the default for the exec setting
- described when workers are removed from cluster.workers
- described addressType, which was documented as existing, but not what
  values it might have
- spell out more clearly the limitations of setupMaster
- describe disconnect in sufficient detail that why a child does or does
  not exit can be understood
- clarify which cluster functions and events are available on process or
  just on the worker, as well as which are not available in children,
- don't describe events as the same, when they have receive different
  arguments
- fix misleading disconnect example: since disconnect already calls
  close on all servers, doing it again in the example is a no-op, not
  the "force close" it was claimed to be
- document the error event, not catching it will kill your node
- describe suicide better, it is important, and a bit unintuitive
  (process.exit() is not suicide?)
- use worker consistently throughout, instead of child.
http_parser: expose pause/resume method for parser
http: provide backpressure for pipeline flood

If a client sends a lot more pipelined requests than we can handle, then
we need to provide backpressure so that the client knows to back off.
Do this by pausing both the stream and the parser itself when the
responses are not being read by the downstream client.

Backport of 085dd30
http: add statusMessage

Now the status message can be set via req.statusMessage = 'msg';