Move platform specific to sys/

Move unix/ and win/ to sys/, so that we can use android/ for android
headers without confusion.

app/src/sys/unix/command.c

@@ -0,0 +1,39 @@
+#include "../../command.h"
+
+#include <signal.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+pid_t cmd_execute(const char *path, const char *const argv[]) {
+    pid_t pid = fork();
+    if (pid == -1) {
+        perror("fork");
+        return -1;
+    }
+    if (pid == 0) {
+        execvp(path, (char *const *)argv);
+        perror("exec");
+        _exit(1);
+    }
+    return pid;
+}
+
+SDL_bool cmd_terminate(pid_t pid) {
+    return kill(pid, SIGTERM) != -1;
+}
+
+SDL_bool cmd_simple_wait(pid_t pid, int *exit_code) {
+    int status;
+    int code;
+    if (waitpid(pid, &status, 0) == -1 || !WIFEXITED(status)) {
+        // cannot wait, or exited unexpectedly, probably by a signal
+        code = -1;
+    } else {
+        code = WEXITSTATUS(status);
+    }
+    if (exit_code) {
+        *exit_code = code;
+    }
+    return !code;
+}