sigtest does comparison of two version of API. First is represented as a sigfile,
and a second is taken either from classpath or via reflection. It is sufficient for
TCKs. Sigtest however is also used internally to track changes between product builds.
It would be useful to be able to compare just two jar files. Something like:
   sigtest -cp1 build24.jar -cp2 build25.jar
In other cases it is useful to compare two sigfiles from two releases:
   sigtest -sig1 tck13.sig -sig2 tck14.sig
Generalizing this approach each API source could be represented as one of the following:
- sigfile
- classpath
- reflection
etc.

Note that si,ilar feature was implemented in APICheck tool so some ideas
of command line options can be reused from there.