Introduce create_shifted_argv().

Introduce create_shifted_argv().

In order to create argument vectors for the gengetopt parsers,
filter_common.c and recv_common.c call create_argv() on the given
--filter or --receiver option, then shift the returned array by one
to the right because gengetopt expects the program name in argv[0].

This commit creates a new helper function for this purpose and changes
the two callers to use the new function.