Add GSSAPI IOV MIC functions

Add gss_get_mic_iov, gss_get_mic_iov_length, and gss_verify_mic_iov
functions, which work similarly to the corresponding IOV wrap
functions.  Add a new buffer type GSS_IOV_BUFFER_TYPE_MIC_TOKEN for
the destination buffer.

Most of the internal code for this was already present, and just
needed to be fixed up and adjusted to use the new buffer type for the
MIC token.

ticket: 7705 (new)
Document AEAD and IOV GSSAPI extensions