services/debug/debug: Tweaks
services/debug/debug: Tweaks

(1) If x509.ParseECPrivateKey fails, try with x509.ParsePKCS8PrivateKey.
    The latter is the format used by the Java libraries
    (java.security.interfaces.ECPrivateKey#getEncoded)
    If I could, I would have just stuck with one format by changing
    the "debug delegate" command to also produce something that parses
    with ParsePKCS8PrivateKey, but I was unable to find a function in
    crypto/x509 to marshal EC private keys in a manner that is parseable
    by ParsePKCS8PrivateKey.
(2) Require --key if --blessings is set. In practice, the two are always
    available together.  In theory it is possible to have --blessings
    contain something that works for the principal in the context.T, however
    to avoid messing with the blessing store/blessing roots of an existing
    principal (changes to which might be persisted) disallow that esoteric
    case.

Change-Id: I22e6223ae72c54a6f46a41a603c75e004f57c5d4

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref