arcstat: add 'avail', fix 'free'

arcstat: add 'avail', fix 'free'

The meaning of the `free` field is currently `zfs_arc_sys_free`, which
is the target amount of memory to leave free for the system, and is
constant after booting.

This commit changes the meaning of `free` to arc_free_memory(), the
amount of memory that the ARC considers to be free.

It also adds a new arcstat field `avail`, which tracks
`arc_available_memory()`.

Since `avail` can be negative, it also updates the arcstat script to
pretty-print negative values.

example output:

  $ arcstat -f time,miss,arcsz,c,grow,need,free,avail 1
      time  miss  arcsz     c  grow  need  free  avail
  15:03:02   39K   114G  114G     0     0  2.4G  407M
  15:03:03   42K   114G  114G     0     0  2.1G  120M
  15:03:04   40K   114G  114G     0     0  1.8G  -177M
  15:03:05   24K   113G  112G     0     0  1.7G  -269M
  15:03:06   29K   111G  110G     0     0  1.6G  -385M
  15:03:07   27K   110G  108G     0     0  1.4G  -535M
  15:03:08   13K   108G  108G     0     0  2.2G  239M
  15:03:09   33K   107G  107G     0     0  1.3G  -639M
  15:03:10   16K   105G  102G     0     0  2.6G  704M
  15:03:11  7.2K   102G  102G     0     0  5.1G  3.1G
  15:03:12   42K   103G  102G     0     0  4.8G  2.8G

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10494