ovl: add support for appending lowerdirs one by one

ovl: add support for appending lowerdirs one by one

Add new mount options lowerdir+ and datadir+ that can be used to add
layers to lower layers stack one by one.

Unlike the legacy lowerdir mount option, special characters (i.e. colons
and cammas) are not unescaped with these new mount options.

The new mount options can be repeated to compose a large stack of lower
layers, but they may not be mixed with the lagacy lowerdir mount option,
because for displaying lower layers in mountinfo, we do not want to mix
escaped with unescaped lower layers path syntax.

Similar to data-only layer rules with the lowerdir mount option, the
datadir+ option must follow at least one lowerdir+ option and the
lowerdir+ option must not follow the datadir+ option.

If the legacy lowerdir mount option follows lowerdir+ and datadir+
mount options, it overrides them.  Sepcifically, calling:

  fsconfig(FSCONFIG_SET_STRING, "lowerdir", "", 0);

can be used to reset previously setup lower layers.

Suggested-by: Miklos Szeredi <miklos@szeredi.hu>
Link: https://lore.kernel.org/r/CAJfpegt7VC94KkRtb1dfHG8+4OzwPBLYqhtc8=QFUxpFJE+=RQ@mail.gmail.com/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>