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
Unless the value is somehow cached, I would suggest saving the return from get_value so that you don't have to re-request sysDescr.0 multiple times.  You could then simplify this code to something like:

```
sys_desc = snmp.get_value('sysDesc.0')
@users = []
case sys_desc
when /Windows/
 // walk and add users from the Windows-specific OID
when /Solaris
 // walk and add users from the Solaris-specific OID
...
end/

unless @users.empty?
  @users.sort!
  @users.uniq!
  print_good("#{ip} Found #{@users.size} users: #{@users.join(', ')}")
end