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.