libmount: implement ro[=vfs,fs]

libmount: implement ro[=vfs,fs]

The new mount API allows for specifying whether to use a read-only
setting on the VFS or FS. Let's export this feature to the mount(8)
command line and fstab by adding new optional arguments "fs" and "vfs"
for the readonly mount flags. If nothing is specified, then the
default will be to mount as read-only on both layers for a normal
mount.

 # mount -o ro=vfs /dev/sdc /mnt/test
 ...
 fsopen("ext4", FSOPEN_CLOEXEC)          = 3
 fsconfig(3, FSCONFIG_SET_STRING, "source", "/dev/sdc", 0) = 0
 fsconfig(3, FSCONFIG_CMD_CREATE, NULL, NULL, 0) = 0
 mount_setattr(4, "", AT_EMPTY_PATH, {attr_set=MOUNT_ATTR_RDONLY, attr_clr=0, propagation=0 /* MS_??? */, userns_fd=0}, 32) = 0
 move_mount(4, "", AT_FDCWD, "/mnt/test", MOVE_MOUNT_F_EMPTY_PATH) = 0

 # mount -o ro=fs /dev/sdc /mnt/test
 ...
 fsopen("ext4", FSOPEN_CLOEXEC)          = 3
 fsconfig(3, FSCONFIG_SET_STRING, "source", "/dev/sdc", 0) = 0
 fsconfig(3, FSCONFIG_SET_FLAG, "ro", NULL, 0) = 0
 fsconfig(3, FSCONFIG_CMD_CREATE, NULL, NULL, 0) = 0
 move_mount(4, "", AT_FDCWD, "/mnt/test", MOVE_MOUNT_F_EMPTY_PATH) = 0

 # mount -o ro /dev/sdc /mnt/test
 ...
 fsopen("ext4", FSOPEN_CLOEXEC)          = 3
 fsconfig(3, FSCONFIG_SET_STRING, "source", "/dev/sdc", 0) = 0
 fsconfig(3, FSCONFIG_SET_FLAG, "ro", NULL, 0) = 0
 fsconfig(3, FSCONFIG_CMD_CREATE, NULL, NULL, 0) = 0
 mount_setattr(4, "", AT_EMPTY_PATH, {attr_set=MOUNT_ATTR_RDONLY, attr_clr=0, propagation=0 /* MS_??? */, userns_fd=0}, 32) = 0
 move_mount(4, "", AT_FDCWD, "/mnt/test", MOVE_MOUNT_F_EMPTY_PATH) = 0

The patch improves also the mount(8) man page to provide clearer
information about VFS.

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