MFC r335245-r335248, r335254, r335276, r335298, r335398, r335868, r335883

r335245:
Correct logic error in biosdisk.c:bd_realstrategy()

The wrong condition is used when evaluating the return of disk_ioctl()
This results in reaching the 'We should not get here' branch in most casts

r335246:
biosdisk.c remove redundant variable

rdev and disk serve the same purpose, read the partition table without
the d_offset or d_slice set, so the read is relative to the start of
the disk. Reuse the already initialized disk instead of making another
copy later.

r335247:
biosdisk.c: add missing \n to error message

r335248:
biosdisk.c: fix type in debug printf

r335254:
Avoid reading past the end of the disk in zfsboot.c and biosdisk.c

The GELI boot code rounds reads up to 4k, since the encrypted sectors are
4k, and must be decrypted as a unit. With oddball sized disks (almost
always virtual), this can lead to reading past the end of the disk.

r335276:
gptboot, zfsboot, gptzfsboot: Enable the video and serial consoles early

Normally the serial console is not enabled until /boot.config is read and
we know how the serial console should be configured. Initialize the
consoles early in 'dual' mode (serial & keyboard) with a default serial
rate of 115200. Then serial is re-initialized once the disk is decrypted
and the /boot.config file can be read.

This allows the GELIBoot passphrase to be provided via the serial console.

r335298:
stand/common/disk.c: Update debug printf

This was missed in r330809 because it is compiled out by default

r335398:
Revert r335276

This was causing issues for people booting.
I will likely bring this back as an optional feature, similar to
boot0sio, like gptboot-serial or something.

r335868:
stand/common/disk.c: Read partition table relative to the start of the disk

If a disk is of an oddball size, like the 200mb + 512b used in rootgen.sh,
when disk_open() is called on a GELI encrypted partition, attempts to read
the partition table fail, as they pass through the decryption process which
turns the already plaintext data into jibberish.

When reading the partition table, always pass a slice and partition setting
of -1, and an offset of 0. Setting the slice to -1 prevents a false
positive when checking the slice against the cache of GELI encrypted
slices.

r335883:
stand/common/disk.c: dev->d_offset still needs to be set to 0

With r335868, I thought this was no longer necessary. I was wrong.