Add systemd notification support

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>

Load module first

Function erlang:function_exported/3 looks only for modules already
loaded. So we have to load module first. We can do it either implicitly
by calling any function from the module (and catch for possible
exceptions if no such module available) or explicitly.

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>