gss_krb5: consistently use unsigned for seqnum

Consistently use unsigned (u32 vs. s32) for seqnum.

In get_mic function, send the local copy of seq_send,
rather than the context version.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>