lib: avoid ambiguous call

Pierre Proncheryrequested to merge
khorben/bsddialog:khorben/refresh-call into main

In barbox.c, a call to curses' refresh() is performed, while a local variable is also called refresh. This renames the local variable to trefresh instead, consistently with the related time_t local variables.

This helps fix the build on NetBSD.

Tested on NetBSD/amd64.

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