bsd-user/mmap.c: Implement MAP_EXCL, required by jemalloc in head

bsd-user/mmap.c: Implement MAP_EXCL, required by jemalloc in head

jemalloc requires a working MAP_EXCL. Ensure that no page is double
mapped when specified. In addition, use guest_range_valid_untagged to
test for valid ranges of pages rather than an incomplete inlined version
of the test that might be wrong.

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