support config delimiter in syslog plugin

[abenbachir]

There are softwares out there that still use NUL-terminating characters when sending events through TCP. Firewalls for example are used a lots, and it's impossible to change their behavior because they are closed source. Most Firewall implement RFC3164 or CEF protocol, so using in_syslog plugin comes very usefull than in_tcp plugin.
I'm adding delimiter we can edit the default delimiter ("\n") from configuration, similar to in_tcp plugin.

I made the change in v0.12, I can do the same for v1.x If this change is good to merge.

Something important to know is that NUL(\0) or newline(\n) characters must be used with double-quote in configuration:

<source>
    type syslog
    protocol_type tcp
    delimiter "\0"
</source>

Docs Changes:
I will update documentation when this PR make it

[repeatedly]

v0.12 will merge only security fix so please send this patch to master.

[abenbachir]

So you mean, this change will only make it to v1.x ?

[repeatedly]

Yes.

[abenbachir]

Done

[repeatedly]

Thanks!