Add FreeBSD support

FreeBSD's `procctl` supports a similar death signal facility with
slightly different arguments.

from `procctl(2)`:
```
int procctl(idtype_t idtype, id_t id, int cmd, void *data);

P_PID   Control the process with the process ID id.  id zero is a
        shortcut for the calling process ID.

PROC_PDEATHSIG_CTL  Request the delivery of a signal when the parent of
                    the calling process exits.  idtype must be P_PID and
                    id must be the either caller's pid or zero, with no
                    difference in effect.  The value is cleared for
                    child processes and when executing set-user-ID or
                    set-group-ID binaries.  data must point to a value
                    of type int indicating the signal that should be
                    delivered to the caller.  Use zero to cancel a
                    previously requested signal delivery.

```

pact.c

@@ -1,9 +1,25 @@
+#ifdef __linux__
 #include <sys/prctl.h>
+#elif __FreeBSD__
+#include <sys/procctl.h>
+#endif
+#include <sys/signal.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <errno.h>
 #include <stdio.h>
 
+int do_ctl(int signo) {
+#ifdef __linux__
+    return prctl(PR_SET_PDEATHSIG, signo, 0, 0, 0);
+#elif __FreeBSD__
+    return procctl(P_PID, 0, PROC_PDEATHSIG_CTL, &signo);
+#else
+    # no platform support
+    return -1
+#endif
+}
+
 int main(int argc, char **argv) {
     if(argc < 2)
         return 1;
@@ -16,8 +32,8 @@ int main(int argc, char **argv) {
         cmd = 2;
     }
     if (!signo)
-        signo = 15;
-    if (prctl(PR_SET_PDEATHSIG, signo, 0, 0, 0) < 0) {
+        signo = SIGTERM;
+    if (do_ctl(signo) < 0) {
         perror("prctl");
         return 1;
     }