MFC r260138: MFV r242733:

MFC r260138: MFV r242733:

3306 zdb should be able to issue reads in parallel
3321 'zpool reopen' command should be documented in the man page
and help message

illumos/illumos-gate@31d7e8fa33fae995f558673adb22641b5aa8b6e1

FreeBSD porting notes: the kernel part of this changeset depends
on Solaris buf(9S) interfaces and are not really applicable for
our use.  vdev_disk.c is patched as-is to reduce diverge from
upstream, but vdev_file.c is left intact.

git-svn-id: svn://svn.freebsd.org/base/stable/8@263394 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f