arping: move packet buffer, sigset and struct ifreq to malloced "globals"

arping: move packet buffer, sigset and struct ifreq to malloced "globals"

This way, we can zero them all in one go.
We do malloc() anyway, thus nothing is lost by mallocing "globals"

function                                             old     new   delta
arping_main                                         1683    1686      +3
finish                                               100      86     -14
catcher                                              350     310     -40
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/2 up/down: 3/-54)             Total: -51 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>