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

[Tim Parenti]

In working on my latest proposed patch, I also noticed an outdated error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
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