syslogd: Read configuration outside of init loop

Move all configuration-file code outside of the initialization loop and
into its own set of functions. Create a readconfigfile() to open the
config and call parseconfigfile() to parse it.

The init() function no longer returns if there was a failure to open the
configuration file. Initialization will be finished, using the backup
logging rules: *.ERR to /dev/console and *.PANIC to all logged in users.

Reviewed by: markj, emaste
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41358