generate_precompile script: try to improve reliability

[vtjnash]

TTY objects are inherently unreliable input channels (documented, per posix design),
so try to slow down the rate of input and speed up the rate of output
by watching the output stream more closely, and detecting intermediate errors.

And also just general cleanup some of our IO handling.

[ghost]

I've discovered that I cannot build on master with ENABLE_TIMINGS enabled. When I do, it hangs forever (i haven't let it run more than an hour, to be fair) during Generating precompile statements....

I am building from b471640, but i have also tried building from 4c2e570 and I saw the same result.

I am on macOS, and this is occurring in a freshly cloned julia repo, both with USE_BINARYBUILDER=0 and without.

EDIT: .... i have no idea why i posted this comment here. Haha I'm sorry. It looks like i might've posted in the wrong thread? Please ignore it.

[Keno]

I've looked into this a bit. The part where it drops data is if the TTY is in canonical mode, where it will stop accepting non-special bytes past a line length of 4095 (https://github.com/torvalds/linux/blob/master/drivers/tty/n_tty.c#L1679-L1683).