Introduce logging of privilege checks (AUDIT)

Introduce logging of privilege checks (AUDIT)

Added functionality saves privilege checking responses in systemd
journal. Such entries may be filtered using CYNARA_LOG_TYPE=AUDIT
field. Logging depends on configuration based on environment variable
CYNARA_AUDIT_LEVEL which may take one of following values:
  * NONE - nothing will be saved
  * DENY - only DENY responses will be saved (DEFAULT behaviour)
  * ALLOW - only ALLOW respones will be saved
  * OTHER - other policy types e.g. plugin specific
  * ALL - all above responses will be saved

Change-Id: Iaa46f3c579660784ffe5edc0c2120b822fb0061a