makefile: regenerate getdate.c when missing
Regenerate getdate.c
ntp: create new socket for each client request

Create a new connected client socket before each request and close it
when a valid reply is received.

This is useful when the network configuration is changed and the client
socket should be reconnected, but the old bound address remains valid
and sendmsg() doesn't return with an error.
ntp: remove socket reconnecting

As new client socket is now created for each request, there is no need
to reconnect the socket. This is a partial revert of commit 43cca04c.
ntp: bind socket only when port or address was specified

This removes an unnecessary system call when getting new connected
client socket.
ntp: remove disabled code in prepare_socket()
ntp: allow binding to address that doesn't exist yet
cmdmon: allow binding to address that doesn't exist yet
conf: return port numbers as int
ntp: use one socket with random port when acquisitionport is 0
configure: check for 64-bit time_t
util: fix printing of timestamps when time_t is longer than long
ntp: move packet size asserts to ntp_core
util: add support for other NTP eras

NTP timestamps use only 32 bits to count seconds and the current NTP era
ends in 2036. Add support for converting NTP timestamps from other NTP
eras on systems with 64-bit time_t.

The earliest assumed NTP time is set by the configure script (by default
to 50 years before the date of the build) and earlier NTP timestamps
underflow to the following NTP era.
test: allow tests to be skipped
test: add 008-ntpera
test: fix check in run script