usttrace: use short signal names for busybox compatibility

The kill command in coreutils will accept the short signal name, but
various versions of busybox will not accept the long signal name.  For
compatibility with busybox use the short signal name.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>