[PAR2SET_JOIN] Fix detection code

[PAR2SET_JOIN] Fix detection code

This makes the detection code more exact. This will now properly fail when
a set with the following files comes along:

Name matches:
X.par2
X.vol0+1.par2
X.001
X.002
...

Protection matches:
X.001
X.002
...

This new type will have to be handled in a seperate PAR2Set-derived class.

Signed-off-by: Ira W. Snyder <devel@irasnyder.com>