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

MFC 262331,262487,262495,262523:
ZFS boot support for bhyveload.

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

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

Bring in some userboot changes from the bhyve branch to reduce diffs.

r238966
Bump up the heap size to 1MB. With a few kernel modules, libstand
zalloc and userboot seem to want to use ~600KB of heap space, which
results in a segfault when malloc fails in bhyveload.

r241180
Clarify comment about default number of FICL dictionary cells.

r241153
Allow the number of FICL dictionary cells to be overridden.
Loading a 7.3 ISO with userboot/amd64 takes up 10035 cells,
overflowing the long-standing default of 10000.

Bump userboot's value up to 15000 cells.

Reviewed by: dteske (r238966,241180)
Obtained from: NetApp

Teach userboot to boot from ISO image files.

Reviewed by: ae@, dfr@
Obtained from: NetApp

Add a callback function to userboot.so to fetch a list of environment
variables and pass them to the kernel.

Reviewed by: dfr

Replace all references to loader_callbacks_v1 with loader_callbacks.

Suggested by: grehan@

Bump USERBOOT_VERSION.

Requested by: dfr

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.