* init/parse_job.c, init/job_class.c, init/job_class.h: Added "usage"
stanza which is used by initctl command.
* init/tests/test_parse_job.c: Tests for "usage" stanza
* init/man/init.5: "usage" stanza documentation.
* util/initctl.c, util/man/initctl.8: Added "inictl usage" command.
* util/tests/test_initctl.c: Tests for ""inictl usage" command.