A test added recently checks that when trying to deserialize a NAR with
two files that Unicode-normalize to the same result either succeeds on
Linux, or fails with an "already exists" error on Darwin. However,
failing with an "already exists" error can in fact also happen on Linux,
when using ZFS with the proper utf8 and Unicode normalization options
set.
This commit fixes the issue by not assuming the behavior from the
current system, but just by blindly checking that either one of the two
aforementioned possibilities happen, whether on Darwin or on Linux.
Additionally, we check that the Unicode normalization behaviour of
nix-store is the same as the host file system.
Note: in general, we rely on the OS to tell us if a name is invalid or
if two names normalize in the same way. But for security, we do want
to make sure that we catch '.', '..', slashes and NUL characters. (NUL
characters aren't really a security issue, but since they would be
truncated when we pass them to the OS, it would be canonicity problem.)
On macOS, `mkdir("x/')` behaves differently than `mkdir("x")` if `x` is
a dangling symlink (the formed succeed while the latter fails). So make
sure we always strip the trailing slash.
