tcp_conn: add initialization of sigaction variables

Without the initialization these variables sometimes contain garbage
data in the .sa_flags field, that makes the signal handling be reset
after recieving the specified signal once. This results in the parent
process being killed by a child process.
TestIcmpPing: add option to specify source interface

This commit adds a test option 'iface' to the IcmpPing test module, that
translates to the -I argument of the ping utility. This option was
already present for the Icmp6Ping module, so I added it here as well.