posix: syslog: add support for syslog

This is just a simple wrapper around Zephyr's native log api.

Note: the standard LOG_ERR syslog priority does conflict with
Zephyr's LOG_ERR() macro. This will need to be worked-around on
a case-by-case basis.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>