SNMP fixes for OSX and added OID for enumerating Solaris Users

applied todb's snmp fixs for OSX https://dev.metasploit.com/redmine/issues/7480

added psProcessUsername MIB to snmp_enumusers - to additionally enumerate users on Solaris Systems
I tried this dirty patch here: https://dev.metasploit.com/redmine/issues/7480

According to @hmoore-r7 it wan't a good idea, I can not remember the discussion and the ticket wasn't updated. Sorry. 

Just pointing to have into account @hmoore-r7 feedback here!