Refactor GSS token header parsing

Refactor GSS token header parsing

Rewrite g_verify_token_header() to use k5input and not to handle
two-byte token IDs (which are specific to the krb5 mechanism).  Add a
more general g_get_token_header() which can handle detached wrappers
and cases where the mech is not known in advance.  Adjust all current
callers, and get rid of the duplicate DER parsing code added in commit
b0a2f8a5365f2eec3e27d78907de9f9d2c80505a.

In k5-input.h, split out tag and length parsing into
k5_der_get_taglen(), needed by g_get_token_header().