bsd-user: Add system independent stack, data and text limiting

Eliminate the x86 specific stack stuff in favor of more generic control
over the process size:
    target_maxtsiz  max text size
    target_dfldsiz  initial data size limit
    target_maxdsiz  max data size
    target_dflssiz  initial stack size limit
    target_maxssiz  max stack size
    target_sgrowsiz amount to grow stack
These can be set on a per-arch basis, and the stack size can be set
on the command line. Adjust the stack size parameters at startup.

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