signal handling

while debating the question of more satisfaying keyboard input control (#161 and #163) i had to notice, that WASI up to now doesn't provide any commands to activate signal handlers.

it provides at least one routine to send signals to the parent (wasi_proc_raise), but it's unable to listen and react to incoming signals. that's a significant limitation, because it makes a lot of sense in practice to e.g. provide cleanup routines in applications, which will be called in case of execution abort etc.

beside this most common utilization it's a simple asynchronous communication mechanism, which has to be seen as closely tied to other input and output capabilities and recognized as a nearly indispensable complement to interact with the outside world. although we perhaps don't need all the support for it's use in process management on POSIX platforms, but we should at least try to satisfy the essential requirements of the C standard library.

No implementation files, entry points, or tests are named. Start by reading the discussion in #161 and #163, then compare the requested signal behavior with the linked C standard library requirements. Done would require an agreed scope for WASI signal handling and the corresponding specification work.