Add tests for various connection string issues

Add tests for consistent support of connection strings in frontend
programs as well as proper handling of unusual characters in database
and user names.  These tests were developed for the issues of
CVE-2016-5424.

To allow testing of names with spaces, change the pg_regress
command-line options --create-role and --dbname to split their arguments
by comma only, not space or comma as before.  Only commas were actually
used in existing uses.

Noah Misch, Michael Paquier, Peter Eisentraut