cabinet: FCIAddFile: set defaults in case callback doesn't set some fields

[Dan Kegel]

[This is me not being lazy.]

FCIAddFile relies on a caller-supplied callback to set
date, time, and attributes fields for the new file.
Some apps (e.g. our test suite) might forget to set some of these,
so set default values before calling back.

Also add a comment in the cabinet test suite noting that
we don't seem to exercise that corner of the api, but should.

Found via this valgrind warning:
Syscall param write(buf) points to uninitialised byte(s)
    at 0x40007F2: (within /lib/ld-2.5.so)
    by 0x44E8B0D: WriteFile (file.c:512)
    by 0x45DE23A: fci_write (extract.c:182)
    by 0x45FE1B6: FCIAddFile (fci.c:2558)
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