sys/getenv: Fix getenv_uint64 to properly handle high set bit

[jhibbits]

Summary

strtoq() handles a signed value, so any value with the high bit set will
be treated as negative, and be clamped. Fix this by adding a new
getenv_uquad(), analogue to getenv_quad(), which uses strtouq() instead
of strtoq(). getenv_array() needs more work to deal with uint64_t, and
will be fixed later as needed.

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