* boot-9.scm (file-exists?): Use stat rather than access?, so as to
        follow the effective UID/GID not the real ID.  file-exists? will
        normally be used as a prelude to opening or some other operation, and
        it's the effective ID which will apply there.

For instance, testing before opening is precisely what
documentation.scm and slib.scm use file-exists? for.  And Emacs
file-exists-p uses stat(), I imagine for reasons along these lines.

Speaking of stat, it's actually provided unconditionally isn't it?
Meaning presumably there's no need to test (provided? 'posix) before
using it in file-is-directory? or now in file-exists?.

I'd be a bit inclined to apply this as a fix to the 1.6 branch too.