libblkid: add blkid_probe_set_vfs() for pluggable I/O

libblkid: add blkid_probe_set_vfs() for pluggable I/O

Add public API to set custom VFS I/O operations on a blkid probe.
This allows callers (e.g., systemd fibers) to replace standard
read/write/lseek/open/close/fsync with custom implementations.

The ops struct is copied into a private allocation owned by the
probe. NULL function pointers fall back to standard syscalls.
Passing NULL resets to defaults.

New public symbol: blkid_probe_set_vfs()
New struct in public header: struct ul_vfs_ops (with include guard
shared with include/vfs.h)

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