[fitsbits] New routines for reading string keywords in CFITSIO

[THIERRY FORVEILLE]

> opps, the new routine will have a different name and will be called
> fits_read_str_key (as compared to the old routine name
> fits_read_key_str).  None of the old routines will be removed from
> CFITSIO so as to not break any existing software that use them.
> 
Two routines with very similar names but different calling conventions
sounds like a trap waiting for a distracted programmer ;-) I strongly
recommend using more different names :-)