Implement tox_loop

[Ansa89]

As requested by @iphydf in #131 (comment)

Supersedes #131

CC: @JFreegman @iphydf @cleverca22

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This change is 

[JFreegman]

Reviewed 8 of 9 files at r2.
Review status: all files reviewed at latest revision, 4 unresolved discussions, some commit checks failed.

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toxcore/Messenger.h, line 276 at r2 (raw file):

    bool loop_run;
    void (*loop_begin_cb)(struct Messenger *tox, void *user_data);

Change these to Messenger *m for consistency.

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toxcore/tox.c, line 477 at r2 (raw file):

 * Gathers a list of every network FD activity is expected on
 * @param sockets an array of size tox_fd_count().
 */

1. No reason to abbreviate FD.
2. What is the max_sockets param?
3. What does this function return?

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toxcore/tox.c, line 489 at r2 (raw file):

        sock_t *tmp_sockets = (sock_t *) realloc(*sockets, fdcount * sizeof(sock_t));

        if (tmp_sockets != NULL) {

I'm not sure if I like the idea of realloc() failing silently. It's fine for now, but it's very easy to mess up.

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toxcore/tox.c, line 503 at r2 (raw file):

    TCP_Connections *conns = m->net_crypto->tcp_c;

    for (i = 0; i < conns->tcp_connections_length; i++) {

Just a style suggestion: Put the 'if max_sockets == 0' clause in the loop condition.

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Comments from Reviewable

[Ansa89]

Review status: 5 of 8 files reviewed at latest revision, 4 unresolved discussions, some commit checks failed.

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toxcore/Messenger.h, line 276 at r2 (raw file):

Previously, JFreegman wrote…

Change these to Messenger *m for consistency.

Done.

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toxcore/tox.c, line 477 at r2 (raw file):

Previously, JFreegman wrote…

1. No reason to abbreviate FD.
2. What is the max_sockets param?
3. What does this function return?

Done.

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toxcore/tox.c, line 489 at r2 (raw file):

Previously, JFreegman wrote…

I'm not sure if I like the idea of realloc() failing silently. It's fine for now, but it's very easy to mess up.

Done.

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toxcore/tox.c, line 503 at r2 (raw file):

Previously, JFreegman wrote…

Just a style suggestion: Put the 'if max_sockets == 0' clause in the loop condition.

Done.

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Comments from Reviewable