Fix long certificate filenames in files export

[WorldThirteen]

Fixes --format files exports when certificates contain missing or unusually large Subject Key Identifier values.

The files export now derives certificate filenames as follows:

• use the uppercase hex-encoded Subject Key Identifier when it is 64 characters or shorter
• use the uppercase SHA-1 hash of the raw Subject Key Identifier value when the SKI is longer than 64 characters
• use the uppercase SHA-1 hash of the raw subject public key bit string when the SKI is missing

Previously Mozilla TL had such certificates that were not able to be written into fs due to long filename.

The filename derivation rules are documented in the README.