set default max errs and max warns

javadoc supports -Xmaxerrs and -Xmaxwarns, but the defaults are currently unset.

With the pending arrival of doclint for javadoc, and the potential for a huge number of messages :-( we should set reasonable default limits for the number of errors and warnings that may be generated.

For javac, the limit is 100 (each). That seems a reasonable value for javadoc as well.