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.
diff --git a/include/osmocom/gsm/gsm_utils.h b/include/osmocom/gsm/gsm_utils.h
index 707047a..c9a3101 100644
--- a/include/osmocom/gsm/gsm_utils.h
+++ b/include/osmocom/gsm/gsm_utils.h
@@ -57,8 +57,13 @@
 enum gsm_band gsm_band_parse(const char *mhz);
 
 int gsm_7bit_decode(char *decoded, const uint8_t *user_data, uint8_t length);
+int gsm_7bit_decode_hdr(char *decoded, const uint8_t *user_data, uint8_t length, uint8_t ud_hdr_ind);
 int gsm_7bit_encode(uint8_t *result, const char *data);
 
+int gsm_septets2octets(uint8_t *result, uint8_t *rdata, uint8_t septet_len, uint8_t padding);
+int gsm_septet_encode(uint8_t *result, const char *data);
+uint8_t gsm_get_octet_len(const uint8_t sept_len);
+
 unsigned int ms_class_gmsk_dbm(enum gsm_band band, int ms_class);
 
 int ms_pwr_ctl_lvl(enum gsm_band band, unsigned int dbm);