records with time==0 get "the time of the last valid record", while records
where time goes backwards (compared to the previous record) are skipped.
Also prints a message saying how many records were changed or skipped due
to these checks. Check was inspired by a simpler check in OpenBSD's version.
This is all meant to sidestep problems that Tillman Hodgson noticed with 'ac'
when running sparc64 with 64-bit time_t's. The real problem is whatever is
creating wtmp records with ut_time==0, of course, but I have not yet figured
out what is doing that.
Reviewed by: no screams from freebsd-sparc64 or bde
MFC after: 2 weeks
e3b218cd7b