front: Pass CRC to Play menu (part of fix for unknown game crashes).

littleguy77 adaptation from commit
cfc3af9c2c82d610ba4119f971aa91177c9354bc
front: Pass the responsibility back to the caller if the ROM is unknown.
front: Pass all CRC matches back to caller; let caller handle ambiguity.
front: Always populate MD5 field in RomDetail.

Now possible because null sections are no longer allowed, and we leave
it to the caller to resolve ambiguous CRC matches.
front: Simplify RomDetail constructor (no functional changes).
front: Assume the best in certain corner cases.

littleguy77 note:
These two fields only control the behavior of the front menu.  Better to
be permissive than restrictive in this case.
front: Construct meta-info from assumptions if no other alternative.

This allows unknown games to be launched.
Merge branch 'fix-unknown-rom-crash'