Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

When loader tries to open GPT partition, but partition table is not GPT,
then try automatically detect an appropriate partition type.

PR: kern/172550
Tested by: Ralf Wenk

Add the flags parameter to the disk_open() function and DISK_F_NOCACHE
flag, that disables the caching of partition tables metadata.
Use this flag for floppies in the libi386/biosdisk driver.

Fix disk_cleanup() to work without DISK_DEBUG too.

Almost each time when loader opens a file, this leads to calling
disk_open(). Very often this is called several times for one file.
This leads to reading partition table metadata for each call. To
reduce the number of disk I/O we have a simple block cache, but it
is very dumb and more than half of I/O operations related to reading
metadata, misses this cache.

Introduce new cache layer to resolve this problem. It is independent
and doesn't need initialization like bcache, and will work by default
for all loaders which use the new DISK API. A successful disk_open()
call to each new disk or partition produces new entry in the cache.
Even more, when disk was already open, now opening of any nested
partitions does not require reading top level partition table.
So, if without this cache, partition table metadata was read around
20-50 times during boot, now it reads only once. This affects the booting
from GPT and MBR from the UFS.

Make the loader a bit smarter, when it tries to open disk and the slice
number is not exactly specified. When the disk has MBR, also try to read
BSD label after ptable_getpart() call. When the disk has GPT, also set
d_partition to 255. Mostly, this is how it worked before.

Rework r239232 to unbreak ZFS detection on MBR slices.

Restore the old behaviour. If requested partition is a BSD slice,
but d_partition isn't explicitly set, then try to open BSD label and its
first partition.

Remove colons from the debug message, device name returned by the
disk_fmtdev() already has the colons.

Unbreak booting from the true dedicated disks.
When we open the disk, check the type of partition table, that has
been detected. If this is BSD label, then we assume this is DD mode.

Reported by: dim@

Add more debug messages.

Introduce new API to work with disks from the loader's drivers.
It uses new API from the part.c to work with partition tables.

Update userboot's disk driver to use new API. Note that struct
loader_callbacks_v1 has changed.

With LOADER_MBR_SUPPORT defined and LOADER_GPT_SUPPORT undefined we would
never call disk_openmbr().

Submitted by: avg
MFC after: 3 days

Fix missing return when LOADER_GPT_SUPPORT is defined, but LOADER_MBR_SUPPORT
is not.

MFC after: 3 days

Fix r223695 to compile on architectures which don't use the MBR scheme; wrap
the MBR support in the common part of the loader in #ifdef's and enable it
only for userboot for now.

Add a version of the FreeBSD bootloader which can run in userland, packaged
as a shared library. This is intended to be used by BHyVe to load FreeBSD
kernels into new virtual machines.