MFC r255226: Add sysctl/tunables for various metaslab variables

MFC slacker:	pjd
MFC r261401:

  Fix newsyslog(8) to use the size of the file instead of the blocks it
  takes on disk, as advertised in newsyslog.conf(5).

Approved by:	bapt (mentor, implicit)
MFC r253441: Manually merge part of vendor import r238583 from Illumos