ucode: adjust to latest ucode api

The public libucode api has been revised to return and expect an
uc_program_t pointer instead of a main function reference.

The program (former main function) is also not implicitly released
by uc_vm_execute() anymore.

Adjust the ucode plugin accordingly to match the new requirements.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>