MFC r268464: MFV r268452:

Explicitly mark file removal transactions as "presumed to result
in a net free of space" so they will not fail with ENOSPC.

Illumos issue:	4950 files sometimes can't be removed from a full
		filesystem
MFC r268469: MFV r268453:

Diff reduction against Illumos.
MFC r268470: MFV r268454:

Refresh zpool list for each interval in order to produce fresh
output.

Illumos issue: 4966 zpool list iterator does not update output