Add unpack_bit_str_hex_as_inx macro to pack.h.

RPCs will switch to using bit_str_hex for serialization from bit_fmt,
but receivers still need to access this as a inx formatted array for now.
Collapse this into one macro rather than repeating a bunch of times.

Note that this would be better performing if consolidated into a single
function rather than using unpack_bit_str_hex then bitstr2inx to convert;
although this is a client-side function and less important than avoiding
performance and throughput issues within slurmctld.