Show an error when a DEFAULT value is explicitly encoded

[robstradling]

Example: GoDaddy used to explicitly encode basicConstraints.cA=FALSE, even though FALSE is the default. This caused issues with mozilla::pkix - see https://bugzilla.mozilla.org/show_bug.cgi?id=988633

https://crt.sh/?id=605153&opt=cablint shows what cablint currently makes of the www.digitalocean.com cert mentioned in that Bugzilla bug.

[pzb]

Turns out asn1c has a bug and is emitting default values in DER. I'll see what I can do to work around it.

[pzb]

Checks added for Extension:critical and BasicConstraints:cA. Leaving this open until the root cause is fixed.

[sleevi]

vlm/asn1c#181 will resolve this bug in asn1c - seems it particularly affected BOOLEANs with default values in DER mode