Strings with newlines break configuration serialization

The REST API does not validate the input string in a way that would prevent it from breaking the configuration serialization. Strings with an embedded newlines must be rejected.

Original description:

The following regex

(?x) start #comment

end

is a valid regex with comments enabled.
It should match "startend"
The Maxscale REST API allows creating of such regexes and they function as expected, but the resulting .cnf file ine "/var/lib/maxscale/maxscale/cnf.d/" is invalid, as it contains a new line.

[replication_filter]

type=filter

module=binlogfilter

match=(?#test1)(?x) start #comment

end

exclude=something

rewrite_src=something

rewrite_dest=something

The REST API returns the following JSON for this filter

            "parameters": {

                "match": "(?x) start #comment\nend",

                "exclude": "something",

                "rewrite_src": "something",

                "rewrite_dest": "something"

            }

The discrepancy between the json and .cnf representations means that after a maxscale restart the .cnf will be loaded and maxscale will crash as this is not a valid .cnf file