In a future commit, we will need read_oneliner() to accept flags other
than just `skip_if_empty`. Instead of having an argument for each flag,
teach read_oneliner() to accept the bitfield `flags` instead. For now,
only recognize the `READ_ONELINER_SKIP_IF_EMPTY` flag. More flags will
be added in a future commit.
The result of this is that parallel topics which introduce invocations
of read_oneliner() will still be compatible with this new function
signature since, instead of passing 1 or 0 for `skip_if_empty`, they'll
be passing 1 or 0 to `flags`, which gives equivalent behavior.
Mechanically fix up invocations of read_oneliner() with the following
spatch