Add base64 functions to libkrb5support exports

This was previously not an issue because nothing outside of
libkrb5support used them.
Remove is_udp field in sendto_kdc.c conn_state

This field is redundant with addr.type.

[ghudson@mit.edu: removed extraneous changes; clarified commit
message]
Remove old comments in sendto_kdc.c

The commented code in question is present in set_conn_state_msg_length
which is called immediately after the comments.

[ghudson@mit.edu: clarified commit message]