libmount: add pluggable VFS I/O support

libmount: add pluggable VFS I/O support

Add three new public APIs for pluggable VFS I/O operations:

- mnt_context_set_vfs(cxt, ops) -- set VFS on mount context (owned copy)
- mnt_cache_refer_vfs(cache, vfs) -- set VFS reference on cache (borrowed)
- mnt_table_refer_vfs(tb, vfs) -- set VFS reference on table (borrowed)

The VFS operations are automatically propagated from the context to its
cache and tables when they are created or set.

Convert blkid probe calls in cache.c (read_from_blkid, fstype_from_blkid)
from blkid_new_probe_from_filename() to the VFS-aware 3-step pattern:
blkid_new_probe() + blkid_probe_set_vfs() + blkid_probe_open_device().

Convert mnt_table_parse_file() to use ul_vfs_fopen() for VFS-aware
file opening.

Signed-off-by: Karel Zak <kzak@redhat.com>