This commit switches check_icmp from getopt to getopt_long
to provide long options too and (most importantly) homogenize
option parsing between the different plugins.
The timeout option was redundant in that the runtime
of check_icmp was always limited by the input parameters
and therefore timeout gets removed with this commit to
avoid that confusion.
The rest of the signal handlings was removed too, since
the added complexity does not provide sufficient returns.
If check_icmp gets a signal, it now dies like most other
programs instead of trying to save some things and return a
(arguably wrong) result.