This expands job listing meta and term search to have similar
functionality to WP core's search. As the core's search includes
clauses that are aggregated with ORs and ANDs, the only way to
include meta fields in this search was to rebuild the search query
from scratch.

With this change, get_job_listings_keyword_search generates the
clauses that are generated in WP_Query::parse_search and adds
additional clauses that search the post meta and terms.