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]
Don't reopen the KDB in update_princ_encryption

kdb5_util update_princ_encryption iterates over the KDB and writes
modified entries.  With the DB2 back end, the first write operation
has to upgrade the lock and reopen the DB, which resets the cursor
state.  Take out an explicit write lock before iterating to avoid this
step.

ticket: 7684 (new)
Document krb5_db_iterate restriction on writing