* add 'backend' for backend global initialization/destruction
and common functions.
* WIP: 'file' acts as interface to named files, provides posix
filehandles with and acquire/release function pair.
* 'filehandle' manages posix filehandles internally, refcounted.
* WIP: 'filehandlecache' keeps unused filehandles in a MRU cache
* WIP: test for filehandle management
* improved filedescriptor test
We have 'File' which associates a name with a file and will be used to
offer an interface for the application.
Posix 'Filehandles' which are managed in a 'Filehandlecache' to utilize
a limited number of fd's at optimum.
Internally we use 'Filedescriptor's which manage files, mapping and all
kinds of metadata. Filedescriptors weak referenced by a registry and freed
when the last user vanishes.
