include: add VFS I/O abstraction layer

include: add VFS I/O abstraction layer

Add include/vfs.h with struct ul_vfs_ops — a pluggable I/O operations
table with function pointers matching POSIX signatures (read, write,
open, close, lseek, fsync). NULL function pointers fall back to real
syscalls. The size field enables forward/backward compatible struct
evolution.

Refactor include/all-io.h to use VFS dispatch internally:
  - ul_write_all() and ul_read_all() become macros calling
    __write_all(NULL, ...) and __read_all(NULL, ...)
  - New ul_vfs_write_all() and ul_vfs_read_all() macros pass
    through a VFS ops struct for custom I/O

Addresses: https://github.com/util-linux/util-linux/issues/4308
Signed-off-by: Karel Zak <kzak@redhat.com>