bsd-user: Sync fork_start/fork_end with linux-user

bsd-user: Sync fork_start/fork_end with linux-user

This reorders some of the calls, deduplicates code between branches and,
most importantly, fixes a double end_exclusive call in the parent that
will cause exclusive_context_count to go negative.

Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Pull-Request: https://github.com/qemu-bsd-user/qemu-bsd-user/pull/52
Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>