Sanity check: re-exporting krb5_auth_con_set_req_cksumtype

[ghudson@MIT.EDU]

I'd like to re-export krb5_auth_con_set_req_cksumtype, and perhaps
krb5_auth_con_set_safe_cksumtype for parity.

These functions were prototyped in krb5 1.2, moved to #if KRB5_PRIVATE
in krb5 1.3, and moved out of krb5.h into k5-int.h in krb5 1.7.

set_req_cksumtype is used by kcmd.c to ensure compatibility with
pre-1.7 versions of the rlogin suite which did not correctly handle
keyed checksum types.  I can't find any uses of set_safe_cksumtype.

It's reasonable to use a private function in appl as long as appl is
bundled with the krb5 tree, but now that we're unbundling the
applications for 1.8 we need a cleaner layering.