2.54: Support Cpanel::JSON::XS as other fast XS choice

Support Cpanel::JSON::XS as other fast JSON XS choice.
Cpanel::JSON::XS has a public bugtracker,
is maintained by cPanel Inc,
works also with 5.6.2
does not use common::sense at run-time, only during testing.

Also fix some deprecated names in the testsuite.
Skip non-matching prototype warnings.