Extend public functions

`fontinst.lua` currently starts by copying
- `os_newline_cp`
- `localtexmf()`
- `get_script_name()`
- `dep_install(deps)`

Please would you consider making these available publicly? 

I'd particularly like to have `dep_install()` and `localtexmf()` available so I'm not copying global functions. I can obviously use different names, but I'd prefer not to if there's no good reason. 