perl plugin: Use PERL_NO_GET_CONTEXT.

perl plugin: Use PERL_NO_GET_CONTEXT.

All Perl_* API functions have been replaced with their unprefixed
counterparts. The currently used Perl interpreter is passed along to each
function requiring access to it. This removes the need to access the global
variable keeping the pointer to the Perl interpreter in most cases but the
entry points into the plugin (i.e. the callbacks registered with collectd).

This prepares the plugin to efficiently support multiple interpreters.

Signed-off-by: Sebastian Harl <sh@tokkee.org>