basic/log: split max log level into multiple "realms"

basic/log: split max log level into multiple "realms"

The single log level is split into an array of log levels. Which index in the
array is used can be determined for each compilation unit separately by setting
a macro before including log.h. All compilation units use the same index
(LOG_REALM_SYSTEMD), so there should be no functional change.

v2:
- the "realm" is squished into the level (upper bits that are not used by
  priority or facility), and unsquished later in functions in log.c.

v3:
- rename REALM_PLUS_LEVEL to LOG_REALM_PLUS_LEVEL and REALM to LOG_REALM_REMOVE_LEVEL.