Update semantics of time.now and runtime.nanotime

Nanotime only needs the time since boot, and time.now should cache the
gettimeofday call just once and use the tsc to calcualte the time from
relative to this thereafter.
Make akaros specific implementation of memclr

The default one uses XMM registers, which we don't seem to support
properly yet.  The plan9 just segfaults for some reason, so we can't
just pull that one in either.  I just copied the akaros kernel's version
of memset in for now until we find a better solution.  At any rate, it's
better than just looping over every byte.
Revert "Initialize overflow of hash buckets to nil before use."

With the discovery of a busted memclr function, this "fix" is no longer
needed.  The reason for the busted memory values was due to memclr
messing up, not any real logic in the code anywhere.

This reverts commit 72b09199f91fe5f94c10468c21add0a7a330f9fb.
Add shbang for bash at the top of the script