sar: Allow user to select interfaces to display

sar: Allow user to select interfaces to display

Add new option "--iface=<iface_list>" to allow the user to specify which
network interfaces to display. This option is to be used with options
"-n DEV" and "-n EDEV".
Sample output:

$ sar -n DEV
Linux 4.4.14-200.fc22.x86_64 (home) 05/12/18 _x86_64_ (8 CPU)

10:35:29        IFACE   rxpck/s   txpck/s    rxkB/s    txkB/s [...]
10:35:31           lo      0.00      0.00      0.00      0.00
10:35:31       virbr0      0.00      0.00      0.00      0.00
10:35:31       wlp5s0      0.00      0.00      0.00      0.00
10:35:31    virbr0-nic      0.00      0.00      0.00      0.00
10:35:31       enp6s0     49.00     41.00     29.17      6.47
10:35:33           lo      0.00      0.00      0.00      0.00
10:35:33       virbr0      0.00      0.00      0.00      0.00
10:35:33       wlp5s0      0.00      0.00      0.00      0.00
10:35:33    virbr0-nic      0.00      0.00      0.00      0.00
10:35:33       enp6s0   1033.50    608.50    993.34    116.88
[...]

$ sar -n DEV --iface=virbr0,enp6s0
Linux 4.4.14-200.fc22.x86_64 (home) 05/12/18 _x86_64_ (8 CPU)

10:35:29        IFACE   rxpck/s   txpck/s    rxkB/s    txkB/s [...]
10:35:31       virbr0      0.00      0.00      0.00      0.00
10:35:31       enp6s0     49.00     41.00     29.17      6.47
10:35:33       virbr0      0.00      0.00      0.00      0.00
10:35:33       enp6s0   1033.50    608.50    993.34    116.88
[...]

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>