apt could give a warning for unrecognized "-A" options

Apt defines options of the "-A*" to be options for annotation processors as opposed to the apt tool (or javac). Currently, the apt tool simply ignores these options other than to pass them along. However, it might be helpful to print a warning message if no processor claimed some option. One complication is that this determination has to be done over the entire apt command and not a single run; i.e. we want to support

apt -Aproc1 -Aproc2

where proc1 and proc2 are run on different sets of source files, e.g. proc2 might only be run on files generated by proc1.