Increase WebSocket frame size and improve large read handling

Some WebSocket applications, like [chan_respoke][], require a larger
frame size than the default 8k; this patch bumps the default to 16k.
This patch also fixes some problems exacerbated by large frames.

The sanity counter was decremented on every fread attempt in
ws_safe_read(), regardless of whether data was read from the socket or
not. For large frames, this could result in loss of sanity prior to
reading the entire frame. (16k frame / 1448 bytes per segment = 12
segments).

This patch changes the sanity counter so that it only decrements when
fread() doesn't read any bytes. This more closely matches the original
intention of ws_safe_read(), given that the error message is
"Websocket seems unresponsive".

This patch also properly logs EOF conditions, so disconnects are no
longer confused with unresponsive connections.

 [chan_respoke]: https://github.com/respoke/chan_respoke

Review: https://reviewboard.asterisk.org/r/4431/
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Merged revisions 432236 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 432237 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432238 65c4cc65-6c06-0410-ace0-fbb531ad65f3