syslogd: Move struct definitions into header

[jfree]

Summary

Extract the struct definitions from the source syslogd.c file and move
them into an isolated syslogd.h header file.

This change facilitates the compartmentalization of syslogd into
separate files.

[jfree]

Update after rebase

[markj]

I'm ok with this, a general comment is just that we should seek to minimize syslogd.h, i.e., try to ensure it only contains stuff that really needs to be shared. For example, I'm pretty sure casper doesn't need RCVBUF_MINSIZE?

[jfree]

In D41377#943266, @markj wrote:

I'm ok with this, a general comment is just that we should seek to minimize syslogd.h, i.e., try to ensure it only contains stuff that really needs to be shared. For example, I'm pretty sure casper doesn't need RCVBUF_MINSIZE?

A later patch will split syslogd into multiple files for configuration parsing, message parsing, and log parsing. This is all needed there.