Rename sys_filedescr_t into sys_file_t
Rename module virtmemory.h into vm.h

The module is renamed and moved from "api/platform/virtmemory.h"
into directory "api/memory/vm.h".

The implementation file is renamed from
"C-kern/platform/Linux/virtmemory.c" into "C-kern/platform/Linux/vm.c".
Implement filereader_t

filereader_t replaces instream_t,instream_mmfile_t and instream_factory_t.
It supports only memory mapped I/O for now.
Which is enough to implement utf8scanner_t project and begin transC scanner/compiler.