sigindex: common function to resolve the frequence index

The frequency index returned by code2idx() is not unique and conflicts
need to be resolved. There were different implementations across the
code, RINEX, RTCM, and the raw receiver decoders. Add a common
exported function to implement this resolution.

sigindex(..., opt) allocates an frequency index given a signal code,
returning the allocated signal index on success of -1 on failure.
Conflicts are resolved using the signal priorities that can be
modified by the options in the opt argument, moving an existing
allocation if necessary.

sigindex(..., NULL) uses the same search algorithm as sigindex() but
does not allocate a new entry or check the consistency of code
priorities. Without the signal priorities, with the opt argument NULL,
the allocation ordering could not be determined, so this case is used
for this non-allocating variant.

Apply this resolution per satellite as the set of signals can vary
between satellites.

Update the RINEX, RTCM3, Unicore and Novatel format decoders to use
this new common function.