gss_krb5: create a define for token header size and clean up ptr location

cleanup:
Document token header size with a #define instead of open-coding it.

Don't needlessly increment "ptr" past the beginning of the header
which makes the values passed to functions more understandable and
eliminates the need for extra "krb5_hdr" pointer.

Clean up some intersecting  white-space issues flagged by checkpatch.pl.

This leaves the checksum length hard-coded at 8 for DES.  A later patch
cleans that up.

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