use syslog priority names rather than numbers

The syslog() calls already used LOG_ERR, ..., LOG_INFO but
sd_journal_send() had numeric values, making it difficult to tell
whether the functions agreed on criticality level.  Without doubt
now they do.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
journald does not use openlog()

Signed-off-by: Sami Kerola <kerolasa@iki.fi>