Fix download of jbig2dec

Fixes #613
Update url of freetype-2.3.9

Fixes #611
Add const version of 'Path::base' accessor
Introduce notifications to file-system interface

This patch extends the file-system interface with the ability to monitor
changes of files or directories. The new 'File_system::sigh' function
can be used to install a signal handler for an open node.

The 'ram_fs' server has been enhanced to support the new interface. So
any file or directory changes can now be observed by 'ram_fs' clients.

Fixes #607
nit_fb: Respond to config changes at runtime

This patch enables 'nit_fb' to respond to dynamic changes of its
configuration, in particular the view position and refresh rate.
Add new 'fs_rom' service

The 'fs_rom' service provides files stored on a file system as ROM
modules via the ROM-session interface.

Fixes #606