More zic cleanup.

The workaround the IANA guys chose to get rid of the clang warning
we'd silenced in commit 23ed2ba8 turns out not to satisfy Coverity.
Go back to the previous solution, ie, remove the useless comparison
to SIZE_MAX.  (In principle, there could be machines out there where
it's not useless because ptrdiff_t is wider than size_t.  But the whole
thing is pretty academic anyway, as we could never approach this limit
for any sane estimate of the amount of data that zic will ever be asked
to work with.)

Also, s/lineno/lineno_t/g, because if we accept their decision to start
using "lineno" as a typedef, it is going to have very unpleasant
consequences in our next pgindent run.  Noted that while fooling with
pltcl yesterday.