Augment systat(1) -swap to display large swap space processes

This change updates the systat(1) -swap display to use libprocstat to
obtain and display per-process swap space usage infomation following its
existing swap devise/file statistics. It also incorporates the disk I/O
information from the -vmstat display.

The new screen looks like below with 'systat -swap':

               /0   /1   /2   /3   /4   /5   /6   /7   /8   /9 /10
Load Average   |

Device/Path Size Used |0% /10 /20 /30 /40 / 60\ 70\ 80\ 90\ 100|
ada0s1b 2048M 2034M
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
zvol/sys/tempora 1024M 1015M
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
zvol/sys/swap 1024M 1014M
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Total 4096M 4063M
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Pid Username Command Swap/Total Per-Process Per-System
24153 hiro seamonkey 98M / 1G 7% 2%
23677 hiro xfce4-pane 28M / 81M 34% XXX 0%
23629 hiro xfce4-sess 25M / 118M 21% XX 0%
23681 hiro xfdesktop 20M / 58M 34% XXX 0%
23678 hiro thunar 15M / 43M 36% XXX 0%
23658 hiro at-spi-bus 14M / 23M 63% XXXXXX 0%
23660 hiro gvfsd 12M / 21M 56% XXXXX 0%

Disks ada0 ada1 ada2 cd0 pass0 pass1 pass2 pass3
KB/t 8.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
tps 0 0 0 0 1 0 0 0
MB/s 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
%busy 0 0 0 0 0 0 0 0

Submitted by: Yoshihiro Ota
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D29754