[tz] Updating zic.c to further match Link line field names

In working on my latest proposed patch, I also noticed an outdated error message in zic.c which referred to the FROM field of a Link line. We had updated these field names in comments and documentation in release 2014g to be more descriptive and readily understandable like the parameters of 'ln': https://github.com/eggert/tz/commit/78e3924ea43079a0f3333677a728e0f0ed679cd3 The attached proposed patch further updates the zic.c code to more consistently use these new names throughout. This mostly affects the naming of parameters and locals, but a few macros and struct fields are also updated for consistency. -- Tim Parenti