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

This fixes newsyslog(8) on ZFS with compression enabled to not have
large files compared to the expected rotation size.

  # grep remotes/messages /etc/newsyslog.conf
  /var/log/remote/messages       root:info       640  5     500  *     JC
  # ls -alh /var/log/remote/messages
  -rw-r-----  1 root  info    3.2M Jan 31 20:02 /var/log/remote/messages
  # newsyslog -vN|grep remote/messages
  /var/log/remote/messages <5J>: size (Kb): 464 [500] --> skipping
  # stat -f "st_size: %z st_blocks: %b" /var/log/remote/messages
  st_size: 3372627 st_blocks: 928
  # zfs get -H compressratio zroot/syslogs
  zroot/syslogs    compressratio   3.77x   -

With fix:

  # newsyslog -v | grep remote/messages
  /var/log/remote/messages <5J>: size (Kb): 3338 [500] --> trimming log....

Approved by:	bapt (mentor)
PR:		docs/150877
Reported by:	Joshua Isom <jrisom@gmail.com>
MFC after:	2 weeks
Add back reference to buf(9) removed in r32223 as buf(9) was added
in r42016.

Approved by:	bapt (mentor)