extract-word: Skip coalesced separators in place

Just skip them in place, instead of setting separator=true. We only do
that in a single place (while finding a separator outside of quote or
backslash states) so we don't really need a separate state for it.

Tested that no regressions were introduced in test-extract-word. Ran a
full `make check` and also installed the binaries on a test system and
did not see any issues related to parsing unit files or starting units
after a reboot.