In addition to letting the FDM do the low-level signal watching, this
patch also fixes a bug; multiple SIGCHLDs, be it delivered either through a
signal, or via a signalfd, can be coalesced, like all signals.
This means we need to loop on waitpid() with WNOHANG until there are
no more processes to reap.
This in turn requires a small change to the way reaper callbacks are
implemented.
Previously, the callback was allowed to do the wait(). This was
signalled back to the reaper through the callback’s return value.
Now, since we’ve already wait():ed, the process’ exit status is passed
as an argument to the reaper callback.
The callback for the client application has been updated accordingly;
it sets a flag in the terminal struct, telling term_destroy() that the
process has already been wait():ed on, and also stores the exit
status.
When calling ‘reaper_add()’, the caller can provide a callback. If
non-NULL, the reaper will call the callback to handle the actual
reaping.
If the callback is NULL, or if it returns false, the reaper reaps the
child process.
Use a signalfd to listen for SIGCHLD signals.
When we receive a SIGCHLD over the signalfd, reap all dead children by
looping over all registered child PIDs and call waitpid(WNOHANG) on
them.
