Fix: POPT_ARG_LONGLONG does not exist in popt 1.13

Fix: POPT_ARG_LONGLONG does not exist in popt 1.13

The retry duration parameter is currently declared as a long long,
which popt 1.13 does not support. The support for that type was only
added in popt 1.14.

Using POPT_ARG_LONG instead of the 'long long' variant makes no real
difference in this case (or at least, not enough to justify bumping
the minimal popt version).

This fixes a build failure that was reported on Red Hat 7 where
1.13 is the default version.

Reported-by: Abderrahmane Benbachir <abenbach@ciena.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>