Refactor cm functions in sendto_kdc.c

Move get_curtime_ms and the cm functions near the top of the file
right after structure definitions.  Except for cm_select_or_poll,
define each cm function separately for poll and for select, since the
implementations don't share much in common.  Instead of
cm_unset_write, define cm_read and cm_write functions to put an fd in
read-only or write-only state.  Remove the ssflags argument from
cm_add_fd and just expect the caller to make a subsequent call to
cm_read or cm_write.  Always select for exceptions when using select.
(Polling for exceptions is implicit with poll).

With these changes, we no longer select/poll for reading on a TCP
connection until we are done writing to it.  So in service_tcp_fd,
remove the check for unexpected read events.
Fix leak in kadm5_flush with LDAP KDB

Due to an inverted test in adb_policy_init, kadm5_flush calls
krb5_db_open twice.  With the DB2 KDB module, the second open is a
no-op, but with the LDAP module, a new DB handle is allocated and the
old one is leaked.

[ghudson@mit.edu: rewrote commit message]

ticket: 7897 (new)
target_version: 1.12.2
tags: pullup
Remove adb_policy_init and adb_policy_close

Since f72c3ffa the policy is initialized as part of database.
adb_policy_close is now a no-op, and adb_policy_init just makes sure
the database is initialized.  adb_policy_init is only called from
kadm5_flush, and only if database initialization was successful
beforehand, rendering this call redundant.

Remove adb_policy_init and adb_policy_close and all their references
in the code and documentation.
Fix sendto_kdc.c on OS X after cm refactoring

Treat POLLHUP without POLLIN or POLLOUT as an exception in the poll
version of cm_get_ssflags, to correctly handle TCP connection errors
on OS X.