Fix TODO: Choose a X509V3_EXT_print() flag

Fix TODO: Choose a X509V3_EXT_print() flag

X509V3_EXT_print() was being summoned to print extensions unrelated to
RPKI. The TODO wanted me to pick a suitable flag for extensions unknown
even to libcrypto.

For reference, this is how X509V3_EXT_print() prints an AIA, as a known
extension:

    CA Issuers - URI:rsync://rpki.ripe.net/repository/aca/KpSo3VVK5wEHIJnHC2QHVV3d5mk.cer

This is how X509V3_EXT_print() prints the same AIA, as an unknown
extension, X509V3_EXT_PARSE_UNKNOWN enabled:

        0:d=0  hl=2 l=  82 cons: SEQUENCE
        2:d=1  hl=2 l=  80 cons: SEQUENCE
        4:d=2  hl=2 l=   8 prim: OBJECT            :CA Issuers
       14:d=2  hl=2 l=  68 prim: cont [ 6 ]

This is how X509V3_EXT_print() prints the same AIA, as an unknown
extension, X509V3_EXT_DUMP_UNKNOWN enabled:

    0000 - 30 52 30 50 06 08 2b 06-01 05 05 07 30 02 86 44   0R0P..+.....0..D
    0010 - 72 73 79 6e 63 3a 2f 2f-72 70 6b 69 2e 72 69 70   rsync://rpki.rip
    0020 - 65 2e 6e 65 74 2f 72 65-70 6f 73 69 74 6f 72 79   e.net/repository
    0030 - 2f 61 63 61 2f 4b 70 53-6f 33 56 56 4b 35 77 45   /aca/KpSo3VVK5wE
    0040 - 48 49 4a 6e 48 43 32 51-48 56 56 33 64 35 6d 6b   HIJnHC2QHVV3d5mk
    0050 - 2e 63 65 72                                       .cer

Eh. These are good and all, but they mess up the JSON, so I decided to
dump these as octet strings instead:

        3052305006082b0601050507300286447273796e633a2f2f72706b692e726970
        652e6e65742f7265706f7369746f72792f6163612f4b70536f3356564b357745
        48494a6e484332514856563364356d6b2e636572