BUG/MEDIUM: log: Properly handle client aborts in syslog applet

BUG/MEDIUM: log: Properly handle client aborts in syslog applet

In the syslog applet, when there is no output data, nothing is performed and
the applet leaves by requesting more data. But it is an issue because a
client abort is only handled if it reported with the last bytes of the
message. If the abort occurs after the message was handled, it is ignored.
The session remains opened and inactive until the client timeout is being
triggered. It no such timeout is configured, given that the default maxconn
is 10, all slots can be quickly busy and make the applet unresponsive.

To fix the issue, the best is to always try to read a message when the I/O
handle is called. This way, the abort can be handled. And if there is no
data, we leave as usual.

This patch should fix the issue #2112. It must be backported as far as 2.4.