sector size, instead of later failing with an error about /boot/mbr
not being a multiple of the sector size (since we end up with an
assumed sector size of MAX_SEC_SIZE * 2).
- We query the sector size via an IOCTL anyway, so if that succeeds
use that instead of probing for it via read(2) calls. This fixes
the problem with fdisk failing to operate on at least graid3 and
md(4) devices on kernels with src/sys/geom/geom_dev.c before
rev. 1.90, due to fdisk failing to detect the sector size.
- When detecting the root device allow "/" characters in it, which
happens with e.g. gmirror devices.
Reviewed by: cperciva
MFC after: 1 week
aee85526aa