stream: Don't signal EOF on '' or Buffer(0)

Those values, if passed to the _read() cb, will not signal an EOF.  Only
null or undefined will mark the end of data, and trigger the end event.

However, great care must be taken if you are returning an empty string
or buffer!  There must be some other thing somewhere that will trigger
a read() call, because there will never be a readable event fired later.

This is in preparation for CryptoStreams being ported to streams2, where
it is safe to simply stop reading, because the crypto cycle process will
cause it to read(0) again at some future date.
stream: Don't stop reading on zero-length decoded output

Fixes regression introduced in 7e1cf84c9efd491d72b25968a70656458ecb6b7c
dns, cares: don't filter NOTIMP, REFUSED, SERVFAIL

Report the aforementioned DNS server failure codes as-is, stop c-ares
from reporting them as ARES_ECONNREFUSED.

Reported by Steve Freegard.
gitignore: Ignore release tarballs and shasum files
buffer: optimize Buffer.prototype.toString('hex')

Move the implementation to C++ land. The old JS implementation used
string concatenation, was dog slow and consumed copious amounts of
memory for large buffers. Example:

  var buf = Buffer(0x1000000);  // 16 MB
  buf.toString('hex')           // Used 3+ GB of memory.

The new implementation operates in O(n) time and space.

Fixes #4700.
debugger: Make the debugger timeout configurable

If the NODE_DEBUGGER_TIMEOUT environment variable is set, then use
that as the number of ms to wait for the debugger to start.

This is primarily to work around a race condition that almost never
happens in real usage with the debugger, but happens EVERY FRACKING
TIME when the debugger tests run as part of 'make test'.
buffer: optimize Buffer.prototype.write(s, 'hex')

Move the implementation to C++ land. This is similar to commit 3f65916
but this time for the write() function and the Buffer(s, 'hex')
constructor.

Speeds up the benchmark below about 24x (2.6s vs 1:02m).

  var s = 'f';
  for (var i = 0; i < 26; ++i) s += s;  // 64 MB
  Buffer(s, 'hex');