bsd-user: Allocate guest virtual address space

bsd-user: Allocate guest virtual address space

With reserved_va, mmap.c expects to have pre-allocated host address
space for the entire guest address space.  When combined with the -B
command-line option, ensure that the chosen address does not overlap
anything else.  Ensure that mmap_next_start is within reserved_va,
as we use it within mmap.c without checking.

Reviewed by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230727161148.444988-1-richard.henderson@linaro.org>