MFC r315732:

Add forward declaration for struct vnode.

The coredump() function provided by this header file has struct vnode *
as an argument.

MFC r298931, r298981, r299375:

Minor spelling fixes in:
sys/dev, sys/sys

Many of these have user-visible strings.

MFC 274817,274878,276801,276840,278976:
Improve support for XSAVE with debuggers.
- Dump an NT_X86_XSTATE note if XSAVE is in use. This note is designed
to match what Linux does in that 1) it dumps the entire XSAVE area
including the fxsave state, and 2) it stashes a copy of the current
xsave mask in the unused padding between the fxsave state and the
xstate header at the same location used by Linux.
- Teach readelf() to recognize NT_X86_XSTATE notes.
- Change PT_GET/SETXSTATE to take the entire XSAVE state instead of
only the extra portion. This avoids having to always make two
ptrace() calls to get or set the full XSAVE state.
- Add a PT_GET_XSTATE_INFO which returns the length of the current
XSTATE save area (so the size of the buffer needed for PT_GETXSTATE)
and the current XSAVE mask (%xcr0).

Revert MFC r266925 because it can lead to instant panic at fexecve():

To allow to run interpreter itself add a new ELF branding type.

Pointed out by: kib, mjg

MFC r266925:

To allow to run the interpreter itself add a new ELF branding type.
Allow Linux ABI to run ELF interpreter.

MFC 274817,274878,276801,276840,278976:
Improve support for XSAVE with debuggers.
- Dump an NT_X86_XSTATE note if XSAVE is in use. This note is designed
to match what Linux does in that 1) it dumps the entire XSAVE area
including the fxsave state, and 2) it stashes a copy of the current
xsave mask in the unused padding between the fxsave state and the
xstate header at the same location used by Linux.
- Teach readelf() to recognize NT_X86_XSTATE notes.
- Change PT_GET/SETXSTATE to take the entire XSAVE state instead of
only the extra portion. This avoids having to always make two
ptrace() calls to get or set the full XSAVE state.
- Add a PT_GET_XSTATE_INFO which returns the length of the current
XSTATE save area (so the size of the buffer needed for PT_GETXSTATE)
and the current XSAVE mask (%xcr0).

Revert MFC r266925 because it can lead to instant panic at fexecve():

To allow to run interpreter itself add a new ELF branding type.

Pointed out by: kib, mjg

MFC r266925:

To allow to run the interpreter itself add a new ELF branding type.
Allow Linux ABI to run ELF interpreter.