Minor doc format fix.

Submitted by:	Yonghyeon PYUN
Add IO_RANGELOCKED flag for vn_rdwr(9), which specifies that vnode is
not locked, but range is.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Fixes for i/o during coredumping:
- Do not dump into system files.
- Do not acquire write reference to the mount point where img.core is
  written, in the coredump().  The vn_rdwr() calls from ELF imgact
  request the write ref from vn_rdwr().  Recursive acqusition of the
  write ref deadlocks with the unmount.
- Instead, take the range lock for the whole core file.  This prevents
  parallel dumping from two processes executing the same image,
  converting the useless interleaved dump into sequential dumping,
  with second core overwriting the first.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Add kernel option KSTACK_USAGE_PROF to sample the stack depth on
interrupts and report the largest value seen as sysctl
debug.max_kstack_used.  Useful to estimate how close the kernel stack
size is to overflow.

In collaboration with:	Larry Baird <lab@gta.com>
Sponsored by:	The FreeBSD Foundation (kib)
MFC after:	1 week
vt(4): Don't recalculate buffer size if we don't know screen size

When the screen size is unknown, it's set to 0x0. We can't use that as
the buffer size, otherwise, functions such as vtbuf_fill() will fail.

This fixes a panic on RaspberryPi, where there's no vt(4) backend
configured early in boot.

PR:		193981
Tested by:	danilo@
MFC after:	3 days
Slightly reword comment.  Move code, which is described by the
comment, after it.

Discussed with:	bde
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week