bsd-user/bsd-file.h: Meat of the write system calls

Implement write, writev, pwrite and pwritev and connect them to the
system call dispatch routine.

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