configure conditional warning flags, fix some ntpd warnings

add compiler-support check for -Wformat-signedness and -Wsign-conversion in wafhelpers/configure.py

fix -Wformat warnings in ntpd/ntp_util.c, ntpd/refclock_pps.c
fix -Wformat warnings, add default case to diagnostic message in ntpd/refclock_trimble.c