library <stat>: input file buffer size must be dynamic

Since its introduction, our evolved /proc/stat API has
relied on a static buffer of 8192 bytes. This approach
is probably Ok for other /proc files but it would only
accommodate around 100 processors. If such a threshold
were exceeded then this interface could never succeed.

Now days 100 processors doesn't seem at all excessive.

So this commit trades that static buffer for a dynamic
self-tuning one. And since so much former top CPU code
was already rolled into this module, we just stole the
already proven top dynamic buffer management code too.

[ this also meant switching low level unbuffered I/O ]
[ calls to standard library buffered I/O calls. that ]
[ is exactly what <slabinfo> and <diskstats> employ. ]

Reference(s):
. 1st gen readstat introduction
commit a410e236abb47c7c43194e61d0566686f81513af

Signed-off-by: Jim Warner <james.warner@comcast.net>

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