procps free: remove -/+ buffers/cache 增加 available

使用 free -m 命令查看内存使用情况时，发现2个不同版本的CentOS系统显示的结果不同，

系统一：

free -m
total used free shared buffers cached
Mem: 992 926 66 83 118 148
-/+ buffers/cache: 659 333
Swap: 0 0 0 codebye.com

系统二：

free
total used free shared buff/cache available
Mem: 1012720 121728 735008 37724 155984 814736
Swap: 524284 33284 491000

搜索后发现：

free: remove -/+ buffers/cache

With introduction of the 'available' column
and with the latest changes in the 'used' evaluation
the -/+ buffers/cache line became redundant.
The first value duplicates the 'used'
column and the second value has a more accurate
brother called 'available'.
www.codebye.com copyright

新版本中 free 命令已经不显示 -/+ buffers/cache