feat(siginfo)!: extract si_status from siginfo

Problem: there is no way to get the exit status or signal of a child
process when handling SIGCHLD

Solution: expose `siginfo_t`'s `si_status` field in `Process` when
extracting info for SIGCHLD

closes vorner#165

src/low_level/extract.c

@@ -55,3 +55,8 @@ pid_t sighook_signal_pid(const siginfo_t *info) {
 uid_t sighook_signal_uid(const siginfo_t *info) {
     return info->si_uid;
 }
+
+c_int sighook_signal_status(const siginfo_t *info, bool *has_status) {
+    *has_status = info->si_signo == SIGCHLD;
+    return info->si_status;
+}

src/low_level/siginfo.rs

@@ -13,6 +13,7 @@ extern "C" {
     fn sighook_signal_cause(info: &siginfo_t) -> ICause;
     fn sighook_signal_pid(info: &siginfo_t) -> pid_t;
     fn sighook_signal_uid(info: &siginfo_t) -> uid_t;
+    fn sighook_signal_status(info: &siginfo_t, has_status: &mut bool) -> c_int;
 }
 
 // Warning: must be in sync with the C code
@@ -66,6 +67,9 @@ pub struct Process {
 
     /// The user owning the process.
     pub uid: uid_t,
+
+    /// The exit status of the process, or the signal number that caused it to change state.
+    pub status: Option<c_int>,
 }
 
 impl Process {
@@ -78,9 +82,12 @@ impl Process {
      * and `si_uid` filled in.
      */
     unsafe fn extract(info: &siginfo_t) -> Self {
+        let mut has_status = false;
+        let status = sighook_signal_status(info, &mut has_status);
         Self {
             pid: sighook_signal_pid(info),
             uid: sighook_signal_uid(info),
+            status: has_status.then_some(status),
         }
     }
 }