etags: In Prolog functions, don't assume int fits in size_t.

etags: In Prolog functions, don't assume int fits in size_t.

This avoids a warning with gcc -Wstrict-overflow.
* etags.c (Prolog_functions, prolog_pr, prolog_atom): Use size_t,
not int, to store sizes.
(prolog_atom): Return 0, not -1, on error.  All callers changed.