Fortify and optimize file-sending

Do this in two ways:

1) only allow ratox to stay in the file-sender for a certain amount
of time
2) Stop hammering tox_file_send_data(). When it returns -1, we put
the given friend into a cooldown-state, because all internal transmission
slots are full.

File sending thus now works in bursts, reading from file_in as long
as tox_do() allows or until tox_file_send_data() fails.

An easy way to see why we need to do the former is piping /dev/urandom
to file_in, which never blocks. Effectively, the user goes "offline"
after a while given he is trapped inside the loop.

Piping to /dev/urandom is not an unrealistic testcase. Imagine a
researcher who desperately needs true random data from his special
RNG in his lab using ratox and piping it through /dev/urandom.

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