No need to keep static port boundaries global, they are not used
directly. Keeping them local also allows to safely reduce their names to
the minimum. Assign hardcoded fallback values also if fscanf() fails.
Get rid of unnecessary braces around return parameter.

Instead of more or less duplicating is_ephemeral() in run_ssfilter(),
simply call the function instead.
Signed-off-by: Phil Sutter <phil@nwl.cc>