Disable libuv worker thread on mac

[brson]

On mac, every libuv event loop creates another thread in order to do something tty-related. This is really expensive for us, so we need to find a way to not do it.

Some IRC logs:

16:01 < brson> on mac (at least) the event loop creates another worker thread. what is it for? does it exist on all platforms?
16:01 <@tjfontaine> brson: that's used for some of the tty checking code, iirc
16:02 <@tjfontaine> brson: afaik it's a mac specific work around
16:02 < brson> tjfontaine: thanks. do you know if it can be disabled?
16:04 <@tjfontaine> brson: looking at it, it doesn't seem so, https://github.com/joyent/libuv/blob/master/src/unix/tty.c#L40-L44 which calls 
                    https://github.com/joyent/libuv/blob/master/src/unix/stream.c#L284-L371

[brson]

16:43 < bnoordhuis> brson: oh, that's probably fedor's fsevents thread
16:46 < bnoordhuis> brson: yes, it's used to drive the CFRunLoop that in turn powers fsevents
16:47 < bnoordhuis> kind of lame that it gets created unconditionally, i wonder why i didn't notice that during review...
16:47 -!- c4milo [~c4milo@207-38-136-210.c3-0.avec-ubr2.nyr-avec.ny.cable.rcn.com] has joined #libuv
16:48 < brson> lazy initialization of that thread would be sweet :)
16:49 < bnoordhuis> yeah. i'll file an issue
16:50 <@isaacs> bnoordhuis: so... yeah.  we should probably leave it as-is.
16:51 < bnoordhuis> isaacs: 'leave as-is' meaning?
16:52 <@isaacs> bnoordhuis: keep as utf8

[brson]

This is fixed upstream - the thread is initialized lazily.