vfs: ROM file system
libc: Add fd status and flags to File_descriptor

Those values need to be stored local to the FD. Including
them in the 'File_descriptor' eases the handling of libc
plugins.
libc: Make File_descriptor::fd_path char const *
Add libc_vfs plugin and test

Issue #999
Retire libc_log, use libc_vfs instead (WiP)
fixup: retire libc_log
fixup: retire libc_log (mupdf)