Handle the driftfile option correctly when ntpd_flags is empty.

The logic I originally wrote to detect whether a driftfile option was in the
set of flags was based on the result of removing the pattern *flag* being an
empty string. That didn't handle the case where the string was empty to
begin with. Doh! So now it also specifically checks for an empty string.

The result of the bad check was that ntpd would run without a driftfile, but
it would do so only if it was running as root instead of the non-priveleged
ntpd user, which isn't a typical case. Ntpd runs fine without a driftfile,
although it does take it longer to stabilize the clock frequency at startup.

Reported by: avg@
Pointy hat: ian@
MFC after: some testing