Invalid jar file signature triggers SecurityException.
Skip invalid file and try to mount other files.

Fixes accessing unsigned data files in Xlet home directory.
Class loader checks signatures for code.

Some code paths in JRE use it directly

- always compare canonical paths
- BD-J core can delete Xlet files

Default Java security policy does not seem to deny reading these files.

Image fetcher tries to use "main" thread group instead of Xlet thread group.
Failure is not fatal.

Java default policy denies all unknown permissions.

Access should be limited based on Xlet properties, but that would be
useless as we don't check signatures yet.

Use system policy to check all other property permissions.

Avoid dumping useless exception when initializatiion failed

Not likely, but possible (failed malloc would cause this).

Java can use 8.3 file name convention when asked for the OS temporary folder.