fmt_asn1dertag encodes an unsigned integer using the ASN.1 DER for
encoding tag. This takes one byte for every 7 bits in the number.
If dest equals FMT_LEN (i.e. is NULL), fmt_asn1dertag returns the
number of bytes it would have written.
For convenience, fmt.h defines the integer FMT_ASN1TAG to be big
enough to contain every possible fmt_asn1dertag output.