clean clang compile, prototypes C23 issue #301

[webisu]

#301 wgrib2 doesn't compile cleanly using clang
tested with AMD clang version 16.0.3 (CLANG: AOCC_4.2.0-Build#89 2023_12_13)
Some old obsolete features are either removed or deprecated from the working draft of C23:
.. K&R function definitions
Wgrib2 had problems and used the old style prototyping for

1. gctpc which is an old library for geolocation
2. prototyping to pointers to variadic functions

This fix addresses #309 (gcc 15) but wasn't tested with gcc 15.

I couldn't get prototyping of pointers to variadic functions to work, so
options which were variadic functions were converted to functions
with a constant number of arguments.

[webisu]

jpeg problem: lost line of code in wgrib2.c while editing.
ipolates problem: when mode == -1, some routines used the fact that sec == NULL
rather than check the value of mode. Set sec = NULL for mode = -1, -2.