memory: skip clearing guest memory that is still zero

The kernel clears every page it hands out, and the backing memory is a
page-file section the host only gives RAM to once a page is touched. So
the clears themselves made every page of a game's heap resident: Mario
Kart World (12 GB layout) had a 12.5 GB working set 20 s after boot.

HostMemory keeps two bits per 64 KiB of backing: used (handed out by the
kernel at least once) and dirty (may hold data). Pages returned to the
kernel mark their granules dirty, and a zero clear only touches dirty
granules; a granule cleared in full is clean again. A new kernel on the
same memory marks everything a previous one used dirty, so leaked
processes can't leak data into the next game. KMemoryManager's
optimized-process fill and KSharedMemory's clear go through the same
path.

Mario Kart World, title screen and attract demo, 180 s: working set 20 s
in 4.7-6.4 GB instead of 12.5 GB, peak 14-17.6 GB instead of 20.1 GB.

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