Terminate custom module scripts on exit

(Fixes #358.)

Subprocesses created for custom module scripts were previously left
running when the parent Waybar process exited. This patch sets the
parent-death signal of child processes (PR_SET_PDEATHSIG on Linux,
PROC_PDEATHSIG_CTL on FreeBSD) to SIGTERM.

Caveats:

* This uses Linux-specific or FreeBSD-specific calls. I don’t know if
  this project targets other systems?
* There is a possibility that Waybar exits after calling `fork()`, but
  before calling `prctl` to set the parent-death signal. In this case,
  the child will not receive the SIGTERM signal and will continue to
  run. I did not handle this case as I consider it quite unlikely, since
  module scripts are usually launched only when Waybar starts. Please
  let me know if you think it needs to be handled.

Testing:

* With `htop` open, run Waybar v0.9.5 with a custom module that has an
  `exec` script. Terminate the Waybar process and notice that the
  script’s subprocess stays alive and is now a child of the init
  process.
* Run Waybar with this patch and follow the same steps as above. Notice
  that this time the script’s subprocess terminates when the parent
  exits.

include/util/command.hpp

@@ -5,6 +5,13 @@
 #include <sys/wait.h>
 #include <unistd.h>
 
+#ifdef __linux__
+#include <sys/prctl.h>
+#endif
+#ifdef __FreeBSD__
+#include <sys/procctl.h>
+#endif
+
 #include <array>
 
 extern std::mutex reap_mtx;
@@ -77,6 +84,18 @@ inline FILE* open(const std::string& cmd, int& pid) {
     // Reset sigmask
     err = pthread_sigmask(SIG_UNBLOCK, &mask, nullptr);
     if (err != 0) spdlog::error("pthread_sigmask in open failed: {}", strerror(err));
+    // Kill child if Waybar exits
+    int deathsig = SIGTERM;
+#ifdef __linux__
+    if (prctl(PR_SET_PDEATHSIG, deathsig) != 0) {
+      spdlog::error("prctl(PR_SET_PDEATHSIG) in open failed: {}", strerror(errno));
+    }
+#endif
+#ifdef __FreeBSD__
+    if (procctl(P_PID, 0, PROC_PDEATHSIG_CTL, reinterpret_cast<void*>(&deathsig)) == -1) {
+      spdlog::error("procctl(PROC_PDEATHSIG_CTL) in open failed: {}", strerror(errno));
+    }
+#endif
     ::close(fd[0]);
     dup2(fd[1], 1);
     setpgid(child_pid, child_pid);