Issue #11: fix printf "%zd" ssize_t warnings on Android

Also use ssize_t instead of 'int' in a few suitable places
Fix intermittent 'routing' test failures

Caused by tests running so slowly that nodes become unreachable due to
timeout before the assertion of reachability is made.  The timeout was
hard coded at five times the tick interval (2.5 seconds).

Introduced new config options: mdp.iftype.*.reachable_timeout_ms and
interface.*.mdp.reachable_timeout_ms to allow the timeout to be
configured.  Configure the timeout to 60 seconds in 'routing' tests.

Changed the config schema to move options interfaces.*.mdp_tick_ms and
interfaces.*.packet_interval into interfaces.*.mdp.tick_ms and
interfaces.*.mdp.packet_interval (introduced new sub-struct 'mdp').