irssinotifier.pl: use perl for sending instead of wget

Running shell programs with unvalidated outside input is scary,
use LWP::UserAgent instead for sending.
irssinotifier.pl: use perl for encrypting insteal of OpenSSL

Running shell programs with unvalidated outside input is scary,
use Crypt::CBC instead for encrypting.
irssinotifier.pl: screen integration

add new option irssinotifier_screen_detached_only to (if we detect
we are running inside screen) only send notifications if screen is
detached.