Usual case for emulation programs, like sde or gdb, is to pass first
arguments to the emulator, and let the guest application handle latter
arguments. To support this behavior, we introduce an option to choose
index of the first argument to be handled by POPL. By default it is 1,
as it is usually corresponds to the binary name.

Add start_index argument to 'parse' function

Replace Visibility with Attribute

The idea is to check option boundaries while parsing to naturally deny
wrong arguments, i.e. passing negative or too big values