controller: Detach shell commands from polybar

Shell commands triggered from action tags used to block polybar until
they finished.

Since we are not actually interested in the output of the commands, it
makes sense to run them completely detached from polybar and have
polybar not block when executing these commands.

Now the spawned child processes no longer get killed when polybar
exits. This is fine because polybar is not responsible for these
processes since they were explicitly started by the user through click
commands.

Ref: #770
Ref: #1680

tests/unit_tests/utils/process.cpp

@@ -0,0 +1,34 @@
+#include "utils/process.hpp"
+
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include <chrono>
+#include <thread>
+
+#include "common/test.hpp"
+
+using namespace polybar;
+using namespace process_util;
+
+TEST(ForkDetached, is_detached) {
+  pid_t pid = fork_detached([] { exec_sh("sleep 0.1"); });
+  int status;
+
+  pid_t res = process_util::wait_for_completion_nohang(pid, &status);
+
+  ASSERT_NE(res, -1);
+
+  EXPECT_FALSE(WIFEXITED(status));
+}
+
+TEST(ForkDetached, exit_code) {
+  pid_t pid = fork_detached([] { exec_sh("exit 42"); });
+  int status = 0;
+  pid_t res = waitpid(pid, &status, 0);
+
+  EXPECT_EQ(res, pid);
+
+  EXPECT_EQ(WEXITSTATUS(status), 42);
+}