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debuglog - backport of util.debuglog() from node v0.11

To facilitate using the `util.debuglog()` function that will be available when node v0.12 is released now, this is a copy extracted from the source.

# require('debuglog')

Return `util.debuglog`, if it exists, otherwise it will return an internal copy of the implementation from node v0.11.

# debuglog(section)

- `section` {String} The section of the program to be debugged
- Returns: {Function} The logging function

This is used to create a function which conditionally writes to stderr based on the existence of a `NODE_DEBUG` environment variable. If the `section` name appears in that environment variable, then the returned function will be similar to `console.error()`. If not, then the returned function is a no-op.

For example:

var debuglog = util.debuglog('foo');
var bar = 123;
debuglog('hello from foo [%d]', bar);

If this program is run with `NODE_DEBUG=foo` in the environment, then it will output something like:

    FOO 3245: hello from foo [123]

where `3245` is the process id. If it is not run with that environment variable set, then it will not print anything.

You may separate multiple `NODE_DEBUG` environment variables with a comma. For example, `NODE_DEBUG=fs,net,tls`.