A consistency check for fil_space_t::name is causing recovery failures
in MDEV-25180 (Atomic ALTER TABLE). So, we'd better remove that field
altogether.

fil_space_t::name was more or less a copy of dict_table_t::name
(except for some special cases), and it was not being used for
anything useful.

There used to be a name_hash, but it had been removed already in
commit a75dbfd7 (MDEV-12266).

We will also remove os_normalize_path(), OS_PATH_SEPARATOR,
OS_PATH_SEPATOR_ALT. On Microsoft Windows, we will treat \ and /
roughly in the same way. The intention is that for per-table
tablespaces, the filenames will always follow the pattern
prefix/databasename/tablename.ibd. (Any \ in the prefix must not
be converted.)

ut_basename_noext(): Remove (unused function).

read_link_file(): Replaces RemoteDatafile::read_link_file().
We will ensure that the last two path component separators are
forward slashes (converting up to 2 trailing backslashes on
Microsoft Windows), so that everywhere else we can
assume that data file names end in "/databasename/tablename.ibd".

Note: On Microsoft Windows, path names that start with \\?\ must
not contain / as path component separators. Previously, such paths
did work in the DATA DIRECTORY argument of InnoDB tables.

Reviewed by: Vladislav Vaintroub