Do not call exit() upon check_no_rrsig() errors

Do not call exit() upon check_no_rrsig() errors

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

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

Define buffer size using a named constant rather than a plain integer.