Add kadmind support for disabling listening

Add kadmind support for disabling listening

Currently setting kdc_listen or kdc_tcp_listen to the empty string
disables listening for UDP and TCP connections respectively, but
setting kadmind_listen or kpasswd_listen to the empty string listens
on the wildcard address.  Make the behavior consistent by changing
loop_add_addresses() to add no listeners when the string contains no
tokens.  Remove the conditionals from the KDC code.

Document the new behavior of kadmind_listen and kpasswd_listen, and
the existing behavior of kdc_listen.

[ghudson@mit.edu: simplified loop_add_addresses(); combined several
commits and rewrote commit message]

ticket: 9151 (new)