extra cases of slash/backslash problem in JarVerifierStream

This relates to an earlier Javasoft defect no 4087128.

The original bug fix sorted out a slash/backslash problem in JarVerifierStream
and added some special cases to allow older broken JARs to be recognised. The
IBM fix added more "broken combinations" than the JavaSoft fix. Here are some
comments from the original defect:

Also the JarVerifierStream works with the "old" format signature entries
by allowing backslash as well as forward slash.
Note that I've allowed for all the combinations of forward/back slash
in the META-INF entry (keeping within what the code allowed for slashes
in the string positionally), namely:

META-INF/ (was already allowed)
/META-INF/ (was already allowed)
META-INF/META-INF\META-INF/
\META-INF