bsd-user: Catch up to run-time reserved_va math

bsd-user: Catch up to run-time reserved_va math

Catch up to linux-user's 8f67b9c694d0, 13c13397556a, 2f7828b57293, and
95059f9c313a by Richard Henderson which made reserved_va a run-time
calculation, defaulting to nothing except in the case of 64-bit host
32-bit target. Also include the adjustment of the comment heading that
work submitted in the same patch stream. Since this is a direct copy,
squash it into one patch rather than follow the Linux evolution since
breaking this down further at this point doesn't make sense for this
"new code".

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>