dhclient(8): default_route_index(): poll(2) -> ppoll(2)

dhclient(8): default_route_index(): poll(2) -> ppoll(2)

Use ppoll(2) instead of poll(2) in default_route_index().  Using
ppoll(2) here forces us to use clock_gettime(2) to measure the
timeout, which is less error-prone than using time(3).

Part of a larger campaign in dhclient(8) to make "time stuff" more
accurate and robust.

Prompted by krw@.  Based on a diff from krw@.

ok krw@