Support ASN.1 encoding without the outer tag

Support ASN.1 encoding without the outer tag

In order to support implicit tagging, make it possible to ASN.1-encode
a value without its outer tag, instead remembering the construction
bit of the omitted tag.

A cleaner design would be to have separate functions for encoding a
value's contents and its tag.  However, we can't do that for atype_fn
or atype_opaque, and the possible indirections between types and
fields mean we want to stay at the "encode everything" level for as
long as possible to allow implicit tagging of the largest possible
subset of types.  If we can get rid of atype_fn, we may be able to
switch to the cleaner design with some adjustments to atype_opaque.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25613 dc483132-0cff-0310-8789-dd5450dbe970