Make sure 'everyone' knows about changes to the sharetab.
+ Instead of opening a temporary sharetab and then renaming
  it, just open it with freopen() writeable and write the info.
  + If sharetab isn't opened, open it with fopen() instead of
    freopen() to avoid segfault if it doesn't exist.
Don't open sharetab in parse_sharetab() - it's already open!
+ Reuse the handle, fseek to beginning and read...