refactor(fs): rename fd_table.rs to fdtable.rs, add error logging and str()

• Rename fd_table.rs -> fdtable.rs, delete set_fd() (unify with set())
• ensure_dir(): log warning instead of silently swallowing errors
• exec(): log warning when ld.so open/read fails
• SyscallErrNo: add str() method for human-readable error names
• SyscallErrNo: add catch-all panic for unknown error codes