Check for truncated source address when receiving packets
ntp: use NTP instead of echo for presend

Switch to NTP for presend as the echo service (RFC 862) is rarely
enabled. When presend is active, send an NTP client packet to the
server/peer and ignore the reply.

This also fixes presend with separate client sockets. The destination
port can't be changed on connected sockets, so the echo packet was sent
to the NTP port instead of the echo port.
ntp: add debug message for received packet
test: add 114-presend
sched: use FD_SETSIZE if defined
sched: check that added file descriptor fits in fd_set