Sap Modules Tidyup

Lots of refactoring of common code into a library. Modules will load when you don't have the library correctly configured but should give you a loud warning when you try and use them.

The NWRFC function calls are blocking, if your command execution (e.g. your reverse shell) doesn't fork then you don't get a response and the global interpreter lock doesn't return meaning MSF will hang. I have modified the gem file to indicate they are blocking and wrapped all calls in a 10 second thread (modifiable in options) which will kill them if they don't return, giving you control back.

* Removes the sap_rfc_dbmcli_sxpg_call_system_command_exec module which was actually a duplicate of sap_soap_rfc_dbmcli_sxpg_call_system_command_exec
* Adds a Python_sxpg_command_exec module
* Adds Python targets to command_exec and call_system exploits to take advantage of X_PYTHON in MAXDB
* Sets the defaults for read_table to read the SM69 contents to find out what commands you can execute.