sms: SMS where cropped (from VTY), concatenation of SMS where not possible

sms: SMS where cropped (from VTY), concatenation of SMS where not possible

Additionally it wasn't possible to send concatenated sms from the vty.
To send multiple sms, it is necessary to use padding bits and add a user_data_header.
Therefore the gsm_7bit_encode function was splitted to gsm_7bit_encode and gsm_septets2octets.
gsm_septets2octets: this is the old gsm_7bit_encode function + additional padding parameter

Additionally the gsm_7bit_decode function was modified to take account for the user_data_header.
With the new gsm_get_octet_len function you can get the octet length for a given septet length.

I also added several sms tests.