Portability fixes for acctdump:
* Do not build on platforms with no <sys/acct.h>
+ * Use different struct acct, and different headers, on different
+ platforms.
+ * Tolerate lack of ac_exitcode.
+ * Tolerate lack of ac_minflt and ac_majflt (and use %lu as is right).
+ * Tolerate variation in ac_flag field name, and, to an extent, value.
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