The original sidebar patch contained a half-implemented attempt to use
realpath() mailbox paths for comparison.  (Presumably so the open mailbox
remains highlighted despite symlink issues).

Add realpath to the Context, and set it when opening a mailbox.
Remove sidebar ifdef for the buffy member, and always set it there too.

Change the sidebar to use the realpath for comparison everywhere.

mutt_buffy_check() is using stat device and inode for comparison.
Perhaps this can be changed to use realpath instead, but that's beyond
the scope of this patch.