MFC r270189:

Add zdb into rescue environment.

On amd64, this would increase the binary size by 1.1MiB and
make it possible to examine zpool status offline, useful for
recovery and diagnostic purposes.

Submitted by:	sef
Obtained from:	FreeNAS
MFC r270239: MFV r270193:

Illumos issues:
    5042 stop using deprecated atomic functions
MFC r270247: MFV r270195:

Illumos issue:
    5045 use atomic_{inc,dec}_* instead of atomic_add_*
MFC r270248: MFV r270196:

Illumos issue:
    5047 don't use atomic_*_nv if you discard the return value