basic/fileio: clean up executable_is_script() a bit

basic/fileio: clean up executable_is_script() a bit

- Rename to script_get_shebang_interpreter and return
  -EMEDIUMTYPE if the executable is not a script.
  We nowadays utilize the scheme of making ret param
  of getters optional, and use them directly as checkers.
- Don't unnecessarily read the whole line, but check
  only the shebang first.