Add compatibility function for fgets(3).

This patch adds the `tor_fgets()` function to our compatibility layer.

`tor_fgets()` adds an additional check for whether the error-bit have
been enabled for the given file stream, if that is the case and `errno`
is set to `EAGAIN` we make sure that we always return NULL.

Unfortunately `fgets(3)` behaves differently on different versions of
the C library.

See: https://bugs.torproject.org/21416
See: https://bugs.torproject.org/20988