The motivation for this is that Perl is moving towards not having
current directory ./ in @INC by default. This is causing
mysql-test-run.pl to fail in latest Debian Unstable:

  https://lists.debian.org/debian-devel-announce/2016/08/msg00013.html

However, we have `use "lib"`, there is no need for current directory
in @INC, except for a gross hack. In mtr_cases.pm, there is a
`require "mtr_misc.pl"`, which hides mtr_misc.pl away in mtr_cases
namespace. And things only work because mysql-test-run.pl loads it
with a different name, `require "lib/mtr_misc.pl"`! (Perl will
`require` only once for each unique filename).

Fix this by only using `require` in main program, and referencing
functions with :: scope from other namespaces. For multi-use in
different namespaces, proper `use` modules should be used.
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>