Bump up stdio cache sizes for major reads

[nickalcock]

(Reported by Leah Neukirchen.)

We routinely read some very large things on almost every invocation (available_filter_functions, kallmodsyms, module.ranges etc). We are doing all this with the default stdio buffer size, which is 1K. We should at least be using PAGE_SIZE: using more than that is risky for /proc but for stuff read out of /lib/modules etc we should probably go all the way to 64K.

This is a bit fiddly since it means we need to manage the stdio buffer lifetimes ourselves, so it's worth trying on one big one in /proc first to see if it has any effect. But it might well be worthwhile in the upcoming performance push.

[ezannoni]

Wait until we have a better idea of where dtrace spends most of its time. We want to do this in a larger context of performance optimization in general.