lib/log level names: disregard SYSLOG_NAMES

lib/log level names: disregard SYSLOG_NAMES

It's a non-standard feature of syslog.h and I don't think it's worth
the trouble.

We didn't really utilize it; someone would have to #define it,
and moreover we would get into problems if this wasn't being done
the same on all places including log.h.
Making the names adapt to the platform would also mean that
config files and docs wouldn't be portable.

Also make kr_log_level2name() return const.