tbf: Add check_polling/set_polling

Currently the checks for and the actual polling is done in several
places by copy & paste of several lines of code. This hinders changes
of they polling is handled internally and also is likely source of
programming mistakes.

Separate this into a check_polling function, that checks whether
polling is possible and returns the FN and the RRBP to be used in
that case. Otherwise the cause is logged (LOGL_DEBUG) and a negative
error value is returned. There are no other side effect beside the
logging.

If the call is successful, the set_polling method can be used to
actually register the polling.

Extend the encoder functions' parameters lists by an rrbp parameter.

Sponsored-by: On-Waves ehf
diff --git a/src/pcu_vty_functions.cpp b/src/pcu_vty_functions.cpp
index 6567962..3fcd3e6 100644
--- a/src/pcu_vty_functions.cpp
+++ b/src/pcu_vty_functions.cpp
@@ -51,7 +51,8 @@
 	vty_out(vty, " created=%lu state=%08x 1st_TS=%d 1st_cTS=%d ctrl_TS=%d "
 			"MS_CLASS=%d/%d%s",
 			tbf->created_ts(), tbf->state_flags, tbf->first_ts,
-			tbf->first_common_ts, tbf->control_ts, tbf->ms_class(),
+			tbf->first_common_ts, tbf->control_ts,
+			tbf->ms_class(),
 			tbf->ms() ? tbf->ms()->egprs_ms_class() : -1,
 			VTY_NEWLINE);
 	vty_out(vty, " TS_alloc=");