aufs: fs_context 6/7, now mount

commit 3450ca9346c07e69e9f6686751ecbd656c61cc30 aufs5-standalone upstream.

This "fs_context" series essentially moves some parts of opts.c to
fsctx.c, and adjusts it to fit fs_context scheme.  No enhancements,
no bugfixes.

This commit makes aufs usable back but "remount" option is still
unusable.

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>