A mirror-supplied LTS codename (field 10 of index.tab) was used verbatim as an alias filename,
so a hostile codename such as `../../../.bashrc` could make nvm_make_alias write outside $NVM_DIR/alias
- with the default layout, into shell startup files.
Constrain remote codenames to safe filename characters at ingestion in nvm_ls_remote_index_tab,
and reject any `..` path component in nvm_make_alias as a backstop for every caller.
