aufs: fs_context 7/7, finally remount

commit 8aa55258925f264b9f5e99167dde63f5e66cd52d 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 finishes the series and makes aufs fully usable back.

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