perl plugin: Made global variables available to Perl plugins.

perl plugin: Made global variables available to Perl plugins.

The "let's become magical" commit.

Each global variable (currently hostname_g and interval_g) will be exported by
introducing an equally named Perl variable. Perl's concept of "magic" is used
to create a read/write interface to the C variables (think of the variables as
being tied). This way any changes to the C variables will be immediately
accessible from the Perl plugin and vice versa.