Poco::FileInputStream broken in 1.12.5 and 1.11.8.

[jbulow]

See 4244c32#r131403165

[obiltschnig]

This will be properly fixed in 1.13.

The main issue is that the original implementation with _defaultMode is broken conceptually.
Poco::FileInputStream, Poco::FileOutputStream and Poco::FileStream should have their own
open() methods with appropriate defaults.

There will be patch releases 1.11.8p1 and 1.12.5p1 with interim fixes to maintain binary compatibility.