Do not call exit() upon verifyset() errors

Do not call exit() upon verifyset() errors

Replace all check_result() and fprintf() calls inside verifyset() with
zoneverify_log_error() calls and error handling code.  Enable
verifyset() to signal errors to the caller using its return value.

Modify the call site of verifyset() so that its errors are properly
handled.

Define buffer sizes using named constants rather than plain integers.