mxqkill: Correct job counts for cancelled groups

Commit 22aadf3 ("sql: Implement mx_update_job2 trigger")
eliminated the `INQ`->`CANCELLED` housekeeping trigger, claiming that
the job state is no longer set to `CANCELLED` by code.

This is incorrect, as update_job_status_cancelled_by_group() still sets
the job status to `CANCELLED` when a group is cancelled.

Modify update_job_status_cancelled_by_group() to set the `job_cancelled`
flag instead of the `job_status`. Leave the job status transition to the
trigger.

Exclude `ASSIGNED` jobs, as the daemon handles them. Also, remove
unnecessary checks for unset `host_hostname`, `server_id`, and
`host_pid`.

mxqkill.c

@@ -130,13 +130,9 @@ static int update_job_status_cancelled_by_group(struct mx_mysql *mysql, struct m
     assert(g->group_id);
 
     stmt = mx_mysql_statement_prepare(mysql,
-            "UPDATE mxq_job SET"
-                " job_status = " status_str(MXQ_JOB_STATUS_CANCELLED)
-                " WHERE group_id = ?"
-                " AND job_status IN (" status_str(MXQ_JOB_STATUS_INQ) "," status_str(MXQ_JOB_STATUS_ASSIGNED) ")"
-                " AND host_hostname = ''"
-                " AND server_id = ''"
-                " AND host_pid = 0"
+            "UPDATE mxq_job SET job_cancelled = TRUE"
+            " WHERE group_id = ?"
+            " AND job_status = " status_str(MXQ_JOB_STATUS_INQ)
             );
     if (!stmt) {
         mx_log_err("mx_mysql_statement_prepare(): %s", mx_mysql_error());