ntp: re-enable server debugging and control by PACKAGECONFIG

The description in a previous patch to disable debugging is incorrect.
Although the option is default-enabled in configure.ac, configure does
respect the option that disables it.

In ntp 4.2.7 the option code is refactored to ntp_debug.m4 and has an
effect in sntp as well.  Adding --disable-debugging to the top-level
configure options overrides the default for both 4.2.6 and 4.2.7 without
patching the distribution.

Make the selection explicit and configurable, but restore the historical
default.  Absence of debugging capability in the server makes it
difficult to validate complex configurations.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>