Support TARGET and TARGET_INDEX in addition to TARGET_NAME

MSP-9228

I never got around to converting targets to using names instead of
indexes, so restore the use of 'Target' to exploit_simple and in
Metasploit::Framework::UI::Console::Exploit::Simple use TARGET_INDEX,
then TARGET, and finally the index of TARGET_NAME to derive the
 #target_index passed as 'Target' to exploit_simple.