Added disable_symlinks directive.

Added disable_symlinks directive.

To completely disable symlinks (disable_symlinks on)
we use openat(O_NOFOLLOW) for each path component
to avoid races.

To allow symlinks with the same owner (disable_symlinks if_not_owner),
use openat() (followed by fstat()) and fstatat(AT_SYMLINK_NOFOLLOW),
and then compare uids between fstat() and fstatat().

As there is a race between openat() and fstatat() we don't
know if openat() in fact opened symlink or not.  Therefore,
we have to compare uids even if fstatat() reports the opened
component isn't a symlink (as we don't know whether it was
symlink during openat() or not).

Default value is off, i.e. symlinks are allowed.