pg_resetwal: Use separate flags for whether an option is given

pg_resetwal: Use separate flags for whether an option is given

Currently, we use special values that are otherwise invalid for each
option to indicate "option was not given". Replace that with separate
boolean variables for each option. It seems more clear to be explicit.

We were already doing that for the -m option, because there were no
invalid values for nextMulti that we could use (since commit
94939c5f3a).

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/81adf5f3-36ad-4bcd-9ba5-1b95c7b7a807@iki.fi