fsm: add ignore_invalid_event_mask bit-mask

An invalid fsm event is an event which is not valid in the current fsm state.
Such invalid events will be logged and osmo_fsm_inst_dispatch() return != 0.
To prevent this log message which could be misleading for the user, the code needed
to add this event to the allstate_event_mask or add it independent to each state.
Or as alternative create a proxy function in front of osmo_fsm_inst_dispatch()
which checks the fsm state before dispatching the signal.
By using ignore_invalid_events the logline can be ignored for certain events
while for others the old behaviour is still preserved.

Related: SYS#6145
Change-Id: Id010ade76de83ccf428f2d18e9f85bcce1d1ea2c
---
M include/osmocom/core/fsm.h
M src/fsm.c
2 files changed, 13 insertions(+), 0 deletions(-)